pgsql: Add tap test for pg_signal_autovacuum role

Add tap test for pg_signal_autovacuum role

This commit provides testig coverage for ccd38024bc3c, checking that a
role granted pg_signal_autovacuum_worker is able to stop a vacuum
worker.

An injection point with a wait is placed at the beginning of autovacuum
worker startup to make sure that a worker is still alive when sending
and processing the signal sent.

Author: Anthony Leung, Michael Paquier, Kirill Reshke
Reviewed-by: Andrey Borodin, Nathan Bossart
Discussion: https://postgr.es/m/CALdSSPiQPuuQpOkF7x0g2QkA5eE-3xXt7hiJFvShV1bHKDvf8w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d2b74882cab84b9f4fdce0f2f32e892ba9164f5c

Modified Files
--------------
src/backend/postmaster/autovacuum.c                |  7 ++
src/test/modules/test_misc/meson.build             |  3 +-
.../modules/test_misc/t/006_signal_autovacuum.pl   | 95 ++++++++++++++++++++++
3 files changed, 104 insertions(+), 1 deletion(-)