gdb/testsuite: improve test regexp in gdb_get_worker_threads

I spotted I made a small mistake in this commit:

  commit aff250145af6c7a8ea9332bc1306c1219f4a63db
  Date:   Fri Nov 24 12:04:36 2023 +0000

      gdb: generate gdb-index identically regardless of work thread count

In this commit I added a new proc in testsuite/lib/gdb.exp called
gdb_get_worker_threads.  This proc uses gdb_test_multiple with two
possible patterns.  One pattern is anchored with '^', while the other
is missing the '^' which it could use.

This commit adds the missing '^'.