Add more tests for relation statistics with rewrites

Add more tests for relation statistics with rewrites

While there are many tests related to relation rewrites, nothing existed
to check how the cumulative statistics behave in such cases for
relations.

A different patch is under discussion to move the relation statistics to
be tracked on a per-relfilenode basis, so as these could be rebuilt
during crash recovery.  This commit gives us a way to check (and perhaps
change) the existing behaviors for several rewrite scenarios, mixing
transactions, sub-transactions, two-phase commit and VACUUM.

Author: Bertrand Drouvot <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Discussion: https://postgr.es/m/[email protected]