Upgrade NUnit to 4.0

[andy840119]

Will have lots of api breaking change after upgrade to nunit 4.0 Some methods moved to LegacyAssert.

see: https://docs.nunit.org/articles/nunit/release-notes/breaking-changes.html#nunit-40

Before start upgrade the nunit, maybe should finish at least one:

• [ ] Wait until osu project upgrade first (?)
• [ ] Understand why nunit make such changes.

[andy840119]

Maybe we should use Assert.That(value, condition) instead?

see: https://github.com/nunit/nunit/issues/2437

[andy840119]

And note that upgrade might have lots of similar breaking changes. Maybe it's time to try Grit