Fix flaky test attribute not working

[smoogipoo]

There's a few issues here:

1. We were throwing arbitrary exceptions, not AssertionExceptions. The latter are handled specially by NUnit to allow retries.
2. If an assertion happens inside [TearDown], it does not re-run the test at all.

Using TestActionAttribute allows us to do basically the same thing without running into the second issue. Note however that this does not do anything like trying to recover the GameHost after a non-assertion exception, so those will still fail immediately and not retry. I think that this is fine and "intentional".

Until steps now throw an AssertionException instead of a TimeoutException. I don't believe we rely on this anywhere. Messaging has also changed a bit:

Previous:

TearDown : System.TimeoutException : "aaaaa" timed out
--TearDown
   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 65

Now:

"aaaaa" timed out
   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

I've run a full osu! test run with this without issue.

[peppy]

This looks promising.