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:

• Adds a configuration option requiring clients to opt in to this behavior (for now, until we change the default).
• Updates the Runner to move the iteration into a helper function
• Adds this helper function within _runTestCase (if opted in)
• Replaces the plan-level iteration with the helper function (if opted out)
• Mirrors the plan-level iteration tests to test-case level iteration tests that test the same conditions with the new behavior

Checklist:

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