Implement FAILING tests

[asashour]

Fixes dart-lang/sdk#58468

To test

Select any current rule test, e.g.

currentlyLinting(); //LINT
currentlyOk(); //OK

Change the expectations to the below, and the tests are still green.

// this is for false positive cases, means it should be ok, but `unfortunately` currently lints
currentlyLinting(); // OK FAILING
// this is for false negative ones, meaning it should lint, but `unfortunately` currently is ok
currentlyOk(); // LINT FAILING

If the FAILING ones actually pass, there is a message printed about the succession.

// this fails the build and gives a message that it actually works
currentlyLinting(); // LINT FAILING
// this fails the build and gives a message that it actually works
currentlyOk(); // OK FAILING

[coveralls]

Coverage decreased (-0.06%) to 93.913% when pulling 590f38c on asashour:2815_failing into def6bf2 on dart-lang:master.

[srawlins]

CC @bwilkerson I think we will be moving to the unit-test-style tests, instead of the e2e tests, especially for cases where we need to mark a "failing test case".

[bwilkerson]

Agreed. That's our plan.