thread_pool: support passing thread IDs

Then use that to fix gem5 error log read race.

run

@@ -715,7 +715,7 @@ Extra options to append at the end of the emulator command line.
                                 if line.rstrip() == self.env['magic_fail_string']:
                                     exit_status = 1
                                     break
-            if exit_status != 0:
+            if exit_status != 0 and self.env['show_stdout']:
                 self.log_error('simulation error detected by parsing logs')
         return exit_status