testing: exit with error if testing.Short is called before flag.Parse

Change-Id: I2fa547d1074ef0931196066678fadd7250a1148d
Reviewed-on: https://go-review.googlesource.com/121936
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/testing/testing.go

@@ -316,6 +316,13 @@ type common struct {
 
 // Short reports whether the -test.short flag is set.
 func Short() bool {
+	// Catch code that calls this from TestMain without first
+	// calling flag.Parse. This shouldn't really be a panic
+	if !flag.Parsed() {
+		fmt.Fprintf(os.Stderr, "testing: testing.Short called before flag.Parse\n")
+		os.Exit(2)
+	}
+
 	return *short
 }