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.

Change-Id: Id9f8c31c5f90614e3f34c63d1a32cf7e9055d68e
Reviewed-on: https://go-review.googlesource.com/16739Reviewed-by: Russ Cox <rsc@golang.org>