runtime: run gdb with a timeout for TestGdbBacktrace

This sometimes times out and we don't have any useful output for
debugging it. Hopefully this will help.

For #37405.

Change-Id: I79074e6fbb9bd16a864c651109a0acbfc8aa6cef
Reviewed-on: https://go-review.googlesource.com/c/go/+/370703


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>