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