Add TAP tests for pg_basebackup with compression

Add TAP tests for pg_basebackup with compression

pg_basebackup is able to use gzip to compress the contents of backups
with the tar format, but there were no tests for that.  This adds a
minimalistic set of tests to check the contents of such base backups,
including sanity checks on the contents generated with gzip commands.
The tests are skipped if Postgres is compiled --without-zlib, and the
checks based on the gzip command are skipped if nothing can be found,
following the same model as the existing tests for pg_receivewal.

Reviewed-by: Georgios Kokolatos
Discussion: https://postgr.es/m/Yb3GEgWwcu4wZDuA@paquier.xyz