Add optional re-run argument to EkatCreateUnitTest

[jgfouca]

Sometimes, an unlucky pick of inputs might lead to a result that exceeds the tolerance of the test. We've decided that, in some cases, it's better to try re-running the test instead of instead of increasing the tolerance.

Something like this:

EkatCreateUnitTest(my_test MAX_RERUN 2 ..) # allows 2 runs