This adds the possibility to address test cases in the
status file with the variant under which the test is running.
This is only allowed in top-level sections.

Example:
[{
  'test-case': [PASS, SLOW],
}]

['variant == foo', {
  'test-case': [FAIL],
}]

The test case "test-case" is marked as slow in all variants.
Additionally, in variant foo, it'll be expected to fail.

This CL also exemplifies the new feature with test cases
running under the ignition_turbofan variant. The
corresponding legacy flag is deprecated.

BUG=v8:5238

Review-Url: https://codereview.chromium.org/2203013002
Cr-Commit-Position: refs/heads/master@{#38342}