feature request: test binaries should support repeating runs

[tamird]

The ability to repeat tests in-process is useful when attempting to reproduce flaky test failures. Go's testing package has the -count flag (https://golang.org/cmd/go/#hdr-Testing_flags), googletest has --gtest_repeat (https://github.com/google/googletest/blob/master/googletest/docs/advanced.md#repeating-the-tests), but rust doesn't seem to have anything.

In a limited test, I ran a test binary 100 times with a filter that excluded all test cases, and that took over 15 seconds. It'd be nice to avoid that overhead.

cc @tmandry