testing: diagnose a potential misuse of RunParallel

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/68030043

src/pkg/testing/benchmark.go

@@ -417,6 +417,9 @@ func (b *B) RunParallel(body func(*PB)) {
 		}()
 	}
 	wg.Wait()
+	if n == 0 {
+		b.Fatal("RunParallel body did not not call PB.Next")
+	}
 }
 
 // SetParallelism sets the number of goroutines used by RunParallel to p*GOMAXPROCS.