[patch] Add process title to test_shm_mq worker

Hi,

albeit just a test module, the test_shm_mq test can run a few seconds,
and during that, it looks like this:

mbanck 2701 - Rsfo 0:00.51 postgres:

With the attached patch, it looks like this:

mbanck 2780 - Rsfo 0:01.63 postgres: test_shm_mq worker

I think the process title got lost in 5373bc2a where bgw_name was
replaced with bgw_type. Maybe that was intentional, but the
corresponding change to src/test/modules/worker_spi was made so that
bgw_name was preserved/changed.

Michael