runtime: suppress thread event prints in gdb test

Pass "set print thread-events off" to gdb to suppress thread
event prints, like "[New Thread 0xe7b83b40 (LWP 18609)]". We
don't check them, and the extra output may confuse our other
checks, in particular, checkCleanBacktrace.

Hopefully fixes #31569.

Change-Id: I6549e1280da7afa1d2e38da2b2fa7cc18c2f0373
Reviewed-on: https://go-review.googlesource.com/c/go/+/172980
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/runtime-gdb_test.go

@@ -180,6 +180,7 @@ func testGdbPython(t *testing.T, cgo bool) {
 	args := []string{"-nx", "-q", "--batch",
 		"-iex", "add-auto-load-safe-path " + filepath.Join(runtime.GOROOT(), "src", "runtime"),
 		"-ex", "set startup-with-shell off",
+		"-ex", "set print thread-events off",
 	}
 	if cgo {
 		// When we build the cgo version of the program, the system's