Speed up pg_regress server readiness testing.

Speed up pg_regress server readiness testing.

Instead of connecting to the server with psql to check if it is ready
for running tests, this changes pg_regress to use PQPing which avoids
performing system() calls which are expensive on some platforms, like
Windows. The frequency of tests is also increased in order to connect
to the server faster.

This patch is part of a larger effort to make testing consume fewer
resources in order to be able to fit more tests into the available
CI system constraints.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20230823192239.jxew5s3sjru63lio@awork3.anarazel.de