runtime: in tests, make sure gdb does not start with a shell

On some systems, gdb is set to: "startup-with-shell on". This
breaks runtime_test. This just make sure gdb does not start by
spawning a shell.

Fixes #15354

Change-Id: Ia040931c61dea22f4fdd79665ab9f84835ecaa70
Reviewed-on: https://go-review.googlesource.com/23142Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/runtime-gdb_test.go

@@ -98,6 +98,7 @@ func TestGdbPython(t *testing.T) {
 
 	args := []string{"-nx", "-q", "--batch", "-iex",
 		fmt.Sprintf("add-auto-load-safe-path %s/src/runtime", runtime.GOROOT()),
+		"-ex", "set startup-with-shell off",
 		"-ex", "info auto-load python-scripts",
 		"-ex", "br main.go:10",
 		"-ex", "run",
@@ -226,6 +227,7 @@ func TestGdbBacktrace(t *testing.T) {
 
 	// Execute gdb commands.
 	args := []string{"-nx", "-batch",
+		"-ex", "set startup-with-shell off",
 		"-ex", "break main.eee",
 		"-ex", "run",
 		"-ex", "backtrace",