gdb/testsuite: handle differences in guile error string output

A new guile test added in commit:

  commit 0a9ccb9d
  Date:   Mon Feb 6 13:04:16 2023 +0000

      gdb: only allow one of thread or task on breakpoints or watchpoints

fails for some versions of guile.  It turns out that some versions of
guile emit an error like this:

  (gdb) guile (set-breakpoint-thread! bp 1)
  ERROR: In procedure set-breakpoint-thread!:
  In procedure gdbscm_set_breakpoint_thread_x: cannot set both task and thread attributes
  Error while executing Scheme code.

while other versions of guile emit the error like this:

  (gdb) guile (set-breakpoint-thread! bp 1)
  ERROR: In procedure set-breakpoint-thread!:
  ERROR: In procedure gdbscm_set_breakpoint_thread_x: cannot set both task and thread attributes
  Error while executing Scheme code.

notice the extra 'ERROR: ' on the second line of output.  This commit
updates the test regexp to handle this optional 'ERROR: ' string.