Add test event marking a test as explicitly eligible for rerun if it is marked flakey.

This will be used in a future change to support rerunning flakey tests
that hit a test result isue in a low-load, single worker test runner phase.

This is implemented as an additive-style event rather than being
evaluated and added to the start_test event because the decorator code
only runs after the start_test event is created and sent.  i.e.
LLDBTestResult.startTest() runs before the test method decorators run.

llvm-svn: 255351