runtime: don't call t.Parallel in TestCgoSignalDeadlock

It seems that when too much other code is running on the system,
the testprogcgo code can overrun its timeouts.

Updates #18598.

Not marking the issue as fixed until it doesn't recur for some time.

Change-Id: Ieaf106b41986fdda76b1d027bb9d5e3fb805cc3b
Reviewed-on: https://go-review.googlesource.com/48233
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/crash_cgo_test.go

@@ -24,7 +24,10 @@ func TestCgoCrashHandler(t *testing.T) {
 }
 
 func TestCgoSignalDeadlock(t *testing.T) {
-	t.Parallel()
+	// Don't call t.Parallel, since too much work going on at the
+	// same time can cause the testprogcgo code to overrun its
+	// timeouts (issue #18598).
+
 	if testing.Short() && runtime.GOOS == "windows" {
 		t.Skip("Skipping in short mode") // takes up to 64 seconds
 	}