runtime: record Python stack on TestGdbPython failure

For #15599.

Change-Id: Icc2e58a3f314b7a098d78fe164ba36f5b2897de6
Reviewed-on: https://go-review.googlesource.com/23481
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/runtime-gdb_test.go

@@ -100,6 +100,7 @@ func TestGdbPython(t *testing.T) {
 		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", "set python print-stack full",
 		"-ex", "br main.go:10",
 		"-ex", "run",
 		"-ex", "echo BEGIN info goroutines\n",