testing: fix bogus benchmark

Fatal must not be called from secondary goroutines.
Fixes #7401.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/67820047

src/pkg/testing/benchmark_test.go

@@ -88,7 +88,6 @@ func TestRunParallelFail(t *testing.T) {
 			// w/o crashing/deadlocking the whole benchmark.
 			b.Log("log")
 			b.Error("error")
-			b.Fatal("fatal")
 		})
 	})
 }