testing: comment out flag.Parse from example

The TestMain docs explain that flag.Parse() should be called if TestMain
itself depends on command-line flags.

The issue here is that the example implementation does not use any
flags, and thus the flag.Parse call is unnecessary. This leads to people
who use this example as a starting point for their own implementations
to forget that the call is not necessary in most cases.

Comment it out instead of removing the line to keep it as a reminder, as
suggested by Minux Ma.

Change-Id: I6ffc5413e7036366ae3cf0f069b7065e832a3b45
Reviewed-on: https://go-review.googlesource.com/33273
Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/testing/testing.go

@@ -196,7 +196,7 @@
 // A simple implementation of TestMain is:
 //
 //	func TestMain(m *testing.M) {
-//		flag.Parse()
+//		// call flag.Parse() here if TestMain uses flags
 //		os.Exit(m.Run())
 //	}
 //