[gdb/testsuite] Fix regexp in list-thread-groups-available.exp

[gdb/testsuite] Fix regexp in list-thread-groups-available.exp

I ran into a gdb.mi/list-thread-groups-available.exp failure:
...
Running gdb/testsuite/gdb.mi/list-thread-groups-available.exp ...
FAIL: gdb.mi/list-thread-groups-available.exp:
        list available thread groups (unexpected output)
PASS: gdb.mi/list-thread-groups-available.exp:
        list available thread groups with filter
...
When doing an experiment of running it 100 times in a row, the failure
reproduced 3 times.

Analyzing the original failure led to insufficient quoting of square brackets
in a regexp.  This patch fixes the regexp, which resulted in 0 failures in a
100-in-a-row run.

Tested on x86_64-linux.

2018-07-19  Tom de Vries  <tdevries@suse.de>

	* gdb.mi/list-thread-groups-available.exp (cores_re): Fix quoting in
	regular	expression.