pgsql: Add more debugging information for bgworker termination tests of

Add more debugging information for bgworker termination tests of worker_spi

widowbird has failed again after af8837a10bc7, with the same symptoms of
a backend still lying around when attempting a database rename with a
bgworker connected to the database being renamed.

We are still not sure yet how the failure can be reached, if this is a
timing issue in the test or an actual bug in the logic used for
interruptible bgworkers.  This commit adds more debugging information in
the backend to help with the analysis as a temporary measure.

Another thing I have noticed is that the queries launching the dynamic
bgworkers or checking pg_stat_activity would connect to the database
renamed.  These are switched to use 'postgres'.  That will hopefully
remove some of the friction of the test, but I doubt that this is the
end of the story.

Discussion: https://postgr.es/m/abtJLEAsf1HZXWdR@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/79a5911fe65be80d4a4cb6ca6f90ae7d2cc6e9bc

Modified Files
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src/backend/postmaster/bgworker.c                     |  6 ++++++
src/test/modules/worker_spi/t/002_worker_terminate.pl | 10 ++++++++--
2 files changed, 14 insertions(+), 2 deletions(-)