testing: add clear panic for duplicate call to t.Parallel

Change-Id: I155633b58e1823344a26c3edf11f5626fae080ee
Reviewed-on: https://go-review.googlesource.com/18204
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/testing/testing.go

@@ -294,6 +294,7 @@ var _ TB = (*B)(nil)
 type T struct {
 	common
 	name          string // Name of test.
+	isParallel    bool
 	startParallel chan bool // Parallel tests will wait on this.
 }
 
@@ -430,6 +431,10 @@ func (t *T) Parallel() {
 	// We don't want to include the time we spend waiting for serial tests
 	// in the test duration. Record the elapsed time thus far and reset the
 	// timer afterwards.
+	if t.isParallel {
+		panic("testing: t.Parallel called multiple times")
+	}
+	t.isParallel = true
 	t.duration += time.Since(t.start)
 	t.signal <- (*T)(nil) // Release main testing loop
 	<-t.startParallel     // Wait for serial tests to finish