[gdb/testsuite] Fix gdb.threads/multi-create-ns-info-thr.exp

[gdb/testsuite] Fix gdb.threads/multi-create-ns-info-thr.exp

With a testsuite setup modified to make expect wait a little bit longer for
gdb output (see PR27957), I reliably run into:
...
PASS: gdb.threads/multi-create-ns-info-thr.exp: continue to breakpoint 1
FAIL: gdb.threads/multi-create-ns-info-thr.exp: continue to breakpoint 2 \
  (timeout)
...

This is due to this regexp:
...
       -re "Breakpoint $decimal,.*$srcfile:$bp_location1" {
...
consuming several lines using the ".*" part, while it's intended to match one
line looking like this:
...
Thread 1 "multi-create-ns" hit Breakpoint 2, create_function () \
  at multi-create.c:45^M
...

Fix this by limiting the regexp to one line.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-06-08  Tom de Vries  <tdevries@suse.de>

	* gdb.threads/multi-create-ns-info-thr.exp: Limit breakpoint regexp to
	one line.