gdb/testsuite: Make hook-stop.exp ignore termination message from GDB stub

When a GDB stub is run via "target remote |", it sometimes produces
extra output that ends up mixed with GDB's own output.  For example,
QEMU's built-in GDB stub responds to the vKill packet by printing

nios2-elf-qemu-system: QEMU: Terminated via GDBstub

before exiting.

This patch fixes the regexp in gdb.base/hook-stop.exp to allow such
messages between GDB's "continuing" and "Inferior killed" messages.

Reviewed-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>