Move test repetition down to the test case level

[harlanhaskins]

This changes the runner such that "repeat until success" or the other repetition policies repeat only the affected test case, rather than repeating the entire test plan.

Fixes #1392 and rdar://130508488

Motivation:

The previous behavior was unexpected and does not match the behavior of XCTest. This changes the behavior to align the behaviors (in the way that users expect).

Modifications:

• Updates the Runner to move the iteration into a helper function, then invokes the helper function from _runTestCase.
• Adds a test to ensure only the specific failing test case is repeated, not every test case.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[harlanhaskins]

Converting to a draft for now; there's follow-on discussions we need to have about behavioral changes before we can actually sink this repetition down