Refactor step buttons + improve assertion messages

[smoogipoo]

I got annoyed with totally useless test failure messages and having to follow the steps to the failures.

A few important changes here:

• I've removed StepButton constructors except where required (UntilStepButton, RepeatStepButton) to preserve behaviour. They now use properties including required and { get; init; } keywords to indicate expected uses.
• UntilStepButton now has a callstack.
• AddStep and AddLabelStep no longer returns the step button. It's not used in o!f and osu!, and I'm not sure where it could ever find a use in practice.
• Every method (e.g. AddUntilStep) calls into the AddStep(StepButton) method which does the scheduling. This is extremely important for generating correct callstacks - it is currently generated inside the lambda which loses context and is why our traces are terrible.

If deemed necessary, I can split this PR up into two. I kinda went down the rabbit hole to see where it ends, so it combines the refactoring.

Demo:

public class SomeTestScene : TestScene
{
    [Test]
    public void FailingAssertStep()
    {
        AddAssert("fail", () => false);
    }

    [Test]
    public void PassingAssertStep()
    {
        AddAssert("pass", () => true);
    }

    [Test]
    public void FailingUntilStep()
    {
        AddUntilStep("fail", () => false);
    }

    [Test]
    public void PassingUntilStep()
    {
        AddUntilStep("pass", () => true);
    }

    [Test]
    public void RepeatStep()
    {
        AddRepeatStep("repeat", () => { }, 10);
    }

    [Test]
    public void WaitStep()
    {
        AddWaitStep("wait", 10);
    }
}

Before:

  Failed FailingAssertStep [81 ms]
  Error Message:
   fail
  Stack Trace:
     at osu.Framework.Threading.ScheduledDelegate.InvokeTask()
   at osu.Framework.Threading.ScheduledDelegate.RunTaskInternal()
   at osu.Framework.Threading.Scheduler.Update()
   at osu.Framework.Graphics.Drawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.updateChild(Drawable c)
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.updateChild(Drawable c)
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.updateChild(Drawable c)
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.updateChild(Drawable c)
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.updateChild(Drawable c)
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.updateChild(Drawable c)
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()
   at osu.Framework.Graphics.Containers.CompositeDrawable.updateChild(Drawable c)
   at osu.Framework.Graphics.Containers.CompositeDrawable.UpdateSubTree()
   at osu.Framework.Platform.GameHost.UpdateFrame()
   at osu.Framework.Platform.HeadlessGameHost.UpdateFrame()
   at osu.Framework.Threading.GameThread.processFrame()
   at osu.Framework.Threading.GameThread.RunSingleFrame()
   at osu.Framework.Threading.GameThread.<createThread>g__runWork|70_0()
   at System.Threading.Thread.StartHelper.Callback(Object state)

---

  Failed FailingUntilStep [10 s]
  Error Message:
   "fail" timed out
  Stack Trace:
     at osu.Framework.Testing.Drawables.Steps.UntilStepButton.<>c__DisplayClass11_0.<.ctor>b__0() in /Users/smgi/Repos/osu-framework/osu.Framework/Testing/Drawables/Steps/UntilStepButton.cs:line 66
   at osu.Framework.Testing.Drawables.Steps.StepButton.PerformStep(Boolean userTriggered) in /Users/smgi/Repos/osu-framework/osu.Framework/Testing/Drawables/Steps/StepButton.cs:line 124
   at osu.Framework.Testing.TestScene.runNextStep(Action onCompletion, Action`1 onError, Func`2 stopCondition) in /Users/smgi/Repos/osu-framework/osu.Framework/Testing/TestScene.cs:line 235
--- End of stack trace from previous location ---
   at osu.Framework.Testing.TestSceneTestRunner.TestRunner.RunTestBlocking(TestScene test) in /Users/smgi/Repos/osu-framework/osu.Framework/Testing/TestSceneTestRunner.cs:line 89
   at osu.Framework.Testing.TestSceneTestRunner.RunTestBlocking(TestScene test) in /Users/smgi/Repos/osu-framework/osu.Framework/Testing/TestSceneTestRunner.cs:line 32
   at osu.Framework.Testing.TestScene.UseTestSceneRunnerAttribute.AfterTest(ITest test) in /Users/smgi/Repos/osu-framework/osu.Framework/Testing/TestScene.cs:line 564
   at NUnit.Framework.Internal.Commands.TestActionCommand.<>c__DisplayClass0_0.<.ctor>b__1(TestExecutionContext context)
   at NUnit.Framework.Internal.Commands.BeforeAndAfterTestCommand.<>c__DisplayClass1_0.<Execute>b__1()
   at NUnit.Framework.Internal.Commands.DelegatingTestCommand.RunTestMethodInThreadAbortSafeZone(TestExecutionContext context, Action action)

After:

  Failed FailingAssertStep [74 ms]
  Error Message:
   fail
  Stack Trace:
     at osu.Framework.Tests.SomeTestScene.FailingAssertStep() in /Users/smgi/Repos/osu-framework/osu.Framework.Tests/SomeTestScene.cs:line 14

---

  Failed FailingUntilStep [10 s]
  Error Message:
   "fail" timed out
  Stack Trace:
     at osu.Framework.Tests.SomeTestScene.FailingUntilStep() in /Users/smgi/Repos/osu-framework/osu.Framework.Tests/SomeTestScene.cs:line 26