Convert sepgsql tests to TAP

The sepgsql tests have not been integrated into the Meson build system 
yet.  I propose to fix that here.

One problem there was that the tests use a very custom construction 
where a top-level shell script internally calls make.  I have converted 
this to a TAP script that does the preliminary checks and then calls 
pg_regress directly, without make.  This seems to get the job done. 
Also, once you have your SELinux environment set up as required, the 
test now works fully automatically; you don't have to do any manual prep 
work.  The whole thing is guarded by PG_TEST_EXTRA=sepgsql now.

Some comments and questions:

- Do we want to keep the old way to run the test?  I don't know all the 
testing scenarios that people might be interested in, but of course it 
would also be good to cut down on the duplication in the test files.

- Strangely, there was apparently so far no way to get to the build 
directory from a TAP script.  They only ever want to read files from the 
source directory.  So I had to add that.

- If you go through the pre-test checks in contrib/sepgsql/test_sepgsql, 
I have converted most of these checks to the Perl script.  Some of the 
checks are obsolete, because they check whether the database has been 
correctly initialized, which is now done by the TAP script anyway.  One 
check that I wasn't sure about is the

# 'psql' command must be executable from test domain

The old test was checking the installation tree, which I guess could be 
set up in random ways.  But do we need this kind of check if we are 
using a temporary installation?

As mentioned in the patch, the documentation needs to be updated.  This 
depends on the outcome of the question above whether we want to keep the 
old tests in some way.