Improve truth tasks

[BraisGabin]

Close #1068

This PR changes a bit the idea behind BuildResultSubject.task. It doesn't return a BuildTaskSubject with an actual == null anymore. Now BuildResultSubject.task fails if the task doesn't exist with a better error message.

Before:

Caused by: value of:
    buildResult.task(:proj:compileJava2)
expected to have a value:
    FAILED
but was:
    null
buildResult was:
    Calculating task graph as no cached configuration is available for tasks: proj:assemble
    > Task :proj:processResources NO-SOURCE
    > Task :proj:compileJava
    > Task :proj:classes
    > Task :proj:jar
    > Task :proj:assemble

    BUILD SUCCESSFUL in 2s
    2 actionable tasks: 2 executed
    Configuration cache entry stored.

After:

Caused by: value of:
    buildResult.getTasks()
expected to contain:
    :proj:compileJava2
but was:
    [:proj:compileJava, :proj:processResources, :proj:classes, :proj:jar, :proj:assemble]
buildResult was:
    Calculating task graph as no cached configuration is available for tasks: proj:assemble
    > Task :proj:processResources NO-SOURCE
    > Task :proj:compileJava
    > Task :proj:classes
    > Task :proj:jar
    > Task :proj:assemble

    BUILD SUCCESSFUL in 2s
    2 actionable tasks: 2 executed
    Configuration cache entry stored.

With only that change it would be impossible to check that a task does not exist. For that reason I added hasNotTask. I'm not very happy with the name, any suggestion is more than welcome.

And this function I think that also improves the error message:

Before:

Caused by: value of:
    buildResult.task(:proj:compileJava)
expected:
    null
but was:
    :proj:compileJava=SUCCESS
buildResult was:
    Calculating task graph as no cached configuration is available for tasks: proj:assemble
    > Task :proj:processResources NO-SOURCE
    > Task :proj:compileJava
    > Task :proj:classes
    > Task :proj:jar
    > Task :proj:assemble

    BUILD SUCCESSFUL in 2s
    2 actionable tasks: 2 executed
    Configuration cache entry stored.

After:

Caused by: value of:
    buildResult.getTasks()
expected not to contain:
    :proj:compileJava
but was:
    [:proj:processResources, :proj:compileJava, :proj:classes, :proj:jar, :proj:assemble]
buildResult was:
    Calculating task graph as no cached configuration is available for tasks: proj:assemble
    > Task :proj:processResources NO-SOURCE
    > Task :proj:compileJava
    > Task :proj:classes
    > Task :proj:jar
    > Task :proj:assemble

    BUILD SUCCESSFUL in 2s
    2 actionable tasks: 2 executed
    Configuration cache entry stored.