runtime: deflake TestPanicRace

The test is inherently racy, and for me fails about 0.05% of the time.
So only fail the test if it fails ten times in a row.

Fixes #20594

Change-Id: I3b3f7598f2196f7406f1a3937f38f21ff0c0e4b5
Reviewed-on: https://go-review.googlesource.com/45020
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/crash_test.go

@@ -579,11 +579,22 @@ func TestPanicRace(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	// The test is intentionally racy, and in my testing does not
+	// produce the expected output about 0.05% of the time.
+	// So run the program in a loop and only fail the test if we
+	// get the wrong output ten times in a row.
+	const tries = 10
+retry:
+	for i := 0; i < tries; i++ {
 		got, err := testEnv(exec.Command(exe, "PanicRace")).CombinedOutput()
 		if err == nil {
-		t.Error("program exited successfully, should have failed")
+			t.Logf("try %d: program exited successfully, should have failed", i+1)
+			continue
 		}
 
+		if i > 0 {
+			t.Logf("try %d:\n", i+1)
+		}
 		t.Logf("%s\n", got)
 
 		wants := []string{
@@ -593,7 +604,13 @@ func TestPanicRace(t *testing.T) {
 		}
 		for _, want := range wants {
 			if !bytes.Contains(got, []byte(want)) {
-			t.Errorf("did not find expected string %q", want)
+				t.Logf("did not find expected string %q", want)
+				continue retry
+			}
 		}
+
+		// Test generated expected output.
+		return
 	}
+	t.Errorf("test ran %d times without producing expected output", tries)
 }