strings: ignore allocation test in cover mode

Fixes #17699

Change-Id: I7ea29a3fc2ca13d9d7e3044cbb8ea22e3435d423
Reviewed-on: https://go-review.googlesource.com/32484
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/strings/strings_test.go

@@ -294,7 +294,7 @@ func TestIndexRune(t *testing.T) {
 			t.Fatalf("'世' at %d; want 4", i)
 		}
 	})
-	if allocs != 0 {
+	if allocs != 0 && testing.CoverMode() == "" {
 		t.Errorf("expected no allocations, got %f", allocs)
 	}
 }