pg_regress: Emit TAP compliant output

This converts pg_regress output format to emit TAP compliant output
while keeping it as human readable as possible for use without TAP
test harnesses. As verbose harness related information isn't really
supported by TAP this also reduces the verbosity of pg_regress runs
which makes scrolling through log output in buildfarm/CI runs a bit
easier as well.

As the meson TAP parser conumes whitespace, the leading indentation
for differentiating parallel tests from sequential tests has been
changed to a single character prefix.

This patch has been around for an extended period of time, reviewers
listed below may have been involved in reviewing a version quite
different from the version in this commit.  The original idea for
this patch was a hacking session with Jinbao Chen.

TAP format testing is also enabled in meson as of this.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Nikolay Shaplov <dhyan@nataraj.su>
Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/BD4B107D-7E53-4794-ACBA-275BEB4327C9@yesql.se
Discussion: https://postgr.es/m/20220221164736.rq3ornzjdkmwk2wo@alap3.anarazel.de

meson.build

@@ -3118,6 +3118,7 @@ foreach test_dir : tests
       env.prepend('PATH', temp_install_bindir, test_dir['bd'])
 
       test_kwargs = {
+        'protocol': 'tap',
         'priority': 10,
         'timeout': 1000,
         'depends': test_deps + t.get('deps', []),