cmd/go: report flaky failures

[jayconrod]

If the fuzz function finds an error for a given input while fuzzing, and the fuzz function finds no error when called again with the same input, go test -fuzz should report that the failure is "flaky".

As of CL 342996, the fuzzing engine will abandon an attempt to minimize the crash if it can't reproduce it. It will still record the unminimized input and exit non-zero though. We should keep doing that. However, the user may find that the crash does not reproduce, delete the input from testdata, then continue instead of investigating the flake.