Improve portability of new worker_spi test

Improve portability of new worker_spi test

The new test 002_worker_terminate relies on the generation of a LOG
entry to check that a worker has been started, but missed the fact that
a node set with log_error_verbosity = verbose would add an error code.
The regexp used for the matching check did not take this case into
account, making the test fail on a timeout.  The regexp is now fixed to
handle the verbose case correctly.

Per buildfarm member prion, that uses log_error_verbosity = verbose.
The error was reproducible by setting this GUC the same way in the test.

Oversight in f1e251be80a0.