On some systems, gdb refuses to load Python plugin from arbitrary
paths, so we have to add $GOROOT/src/runtime to auto-load-safe-path
in the gdb script test.

Change-Id: Icc44baab8d04a65bd21ceac2ab8ddb13c8d083e8
Reviewed-on: https://go-review.googlesource.com/2905Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>