Document how to run specific regress tests

Added a section to regress.sgml describing make check-tests and make
installcheck-tests.

Author: pjungwir

doc/src/sgml/regress.sgml

@@ -125,6 +125,18 @@ make installcheck-parallel
   </para>
   </sect2>
 
+  <sect2 id="regress-subset">
+   <title>Running Specific Tests</title>
+
+   <para>
+    You can run a subset of the regression tests by saying
+    <literal>make check-tests TESTS="boolean char"</literal> or
+    <literal>make installcheck-tests TESTS="boolean char"</literal>.
+    Note that sometimes tests depend on each other, so you might get
+    extra failures.
+   </para>
+  </sect2>
+
   <sect2 id="regress-additional">
    <title>Additional Test Suites</title>