fstests: fix group list generation for whacky test names

Darrick noticed that tests/xfs/191-input-validation didn't get
generated properly. Fix the regex to handle this.

$ grep -I -R "^_begin_fstest" tests/xfs | \
  sed -e 's/^.*\/\([0-9]*\):_begin_fstest/\1/' |grep 191
tests/xfs/191-input-validation:_begin_fstest auto quick mkfs realtime
$
$ grep -I -R "^_begin_fstest" tests/xfs | \
  sed -e 's/^.*\/\([0-9]*\).*:_begin_fstest/\1/ ' |grep 191
191 auto quick mkfs realtime
$

Use the regexes for matching test names defined in common/test_names
rather than trying to open code it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>