pgsql: doc: Document how to run a subset of regress tests

doc: Document how to run a subset of regress tests

This patch was originally submitted a year ago, but never
ended up getting committed. It was later brought up again
on a recent thread on the same subject.

Original patch by Paul A Jungwirth with some wordsmithing
by me based on the review from the original thread.

Author: Paul A. Jungwirth <pj(at)illuminatedcomputing(dot)com>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Reviewed-by: Viktor Holmberg <v(at)viktorh(dot)net>
Reviewed-by: Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA+renyXB5jYG9r5-CaDc4g607EB398QwTk_efEXTzarrO8bPzw@mail.gmail.com
Discussion: https://postgr.es/m/CACJufxHOcmeTkoh2CxFHKv9GRnp9sLVzN=LZhqTgvqT++PXZNQ@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/20bff3d7945ec23b44322232d608f5776a1f0e35

Modified Files
--------------
doc/src/sgml/regress.sgml | 12 ++++++++++++
1 file changed, 12 insertions(+)