testing: only call flag.Parse if it has not been called before
Calling flag.Parse twice can be problematic if other goroutines called
flag.Parsed in between: the race detector complains due to the
write after read from a different goroutine.
This can happen if TestMain calls flag.Parse and launches goroutines
that call flag.Parsed, for example if it initializes a server which
checks flags.
This patch makes testing.M.Run only parse the flags if they have not
been parsed already.