Convert sepgsql tests to TAP

Convert sepgsql tests to TAP

Add a TAP test for sepgsql.  This automates the previously required
manual setup before the test.  The actual tests are still run by
pg_regress, as before, but now called from within the TAP Perl script.

The previous manual test script (test_sepgsql) is left in place, since
its purpose is (also) to test whether a running instance was properly
initialized for sepgsql.  But it has been changed to call pg_regress
directly and no longer require make.

Reviewed-by: Andreas Karlsson 
Discussion: https://www.postgresql.org/message-id/flat/651a5baf-5c45-4a5a-a202-0c8453a4ebf8@eisentraut.org