Some regression tests for the pg_control_*() functions

Hi all,

As mentioned in [1], there is no regression tests for the SQL control
functions: pg_control_checkpoint, pg_control_recovery,
pg_control_system and pg_control_init.

It would be minimal to check their execution, as of a "SELECT FROM
func()", still some validation can be done on its output as long as
the test is portable enough (needs transparency for wal_level, commit
timestamps, etc.).

Attached is a proposal to provide some coverage.  Some of the checks
could be just removed, like the ones for non-NULL fields, but I have
written out everything to show how much could be done.

Thoughts?

[1]: https://www.postgresql.org/message-id/YzY0iLxNbmaxHpbs@paquier.xyz
--
Michael