testing: race in outputting to the root io.Writer

[dsnet]

Using Go1.7.4.

Consider the following test:

func Test(t *testing.T) {
	var wg sync.WaitGroup
	defer wg.Wait()
	for i := 0; i < 100; i++ {
		wg.Add(1)
		go func(i int) {
			defer wg.Done()
			t.Run(fmt.Sprintf("LongLongLongLongName%d", i), func(t *testing.T) {})
		}(i)
	}
}

On some invocations, I see something like:

=== RUN   Test/LongLongLongLongName48
=== RUN   Test/LongLon=== RUN   Test/LongLongLongLongName50
gLongLongName49
=== RUN   Test/LongLongLongLongName51

In testing.go, we directly write "=== RUN %s\n" directly to the root's io.Writer (which is os.Stdout). Although it is safe to concurrently write to os.Stdout, the results can sometimes be non-sensical.

Instead, we should acquire the root's lock before writing.

[gopherbot]

CL https://golang.org/cl/35556 mentions this issue.