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.

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

src/testing/testing.go

@@ -485,7 +485,11 @@ func MainStart(matchString func(pat, str string) (bool, error), tests []Internal
 
 // Run runs the tests. It returns an exit code to pass to os.Exit.
 func (m *M) Run() int {
+	// TestMain may have already called flag.Parse.
+	if !flag.Parsed() {
 		flag.Parse()
+	}
+
 	parseCpuList()
 
 	before()