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

doc/src/sgml/regress.sgml

@@ -125,6 +125,18 @@ make installcheck-parallel
   </para>
   </sect2>
 
+  <sect2 id="regress-subset">
+   <title>Running Specific Tests</title>
+
+   <para>
+    A subset of the regression tests can be run with the command
+    <literal>make check-tests TESTS="boolean char"</literal> or
+    <literal>make installcheck-tests TESTS="boolean char"</literal>.
+    Note that sometimes tests have dependencies on objects created by other
+    tests, which can cause unexpected failures.
+   </para>
+  </sect2>
+
   <sect2 id="regress-additional">
    <title>Additional Test Suites</title>