doc: document the TAP test environment variables

The TAP tests can, to some degree, be controlled by a set of environment
variables. These were however only documented in a README and not in the
main documentation.  This adds documentation of these variables, as well
as changes one CPAN reference to a ulink for consistency.  While there,
also tag CPAN as an acronym as it's listed in the acronyms section.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/YyPd9unV14SX2bLF@paquier.xyz

doc/src/sgml/install-windows.sgml

@@ -487,7 +487,8 @@ $ENV{CONFIG}="Debug";
       As of this writing, <literal>IPC::Run</literal> is not included in the
       ActiveState Perl installation, nor in the ActiveState Perl Package
       Manager (PPM) library. To install, download the
-      <filename>IPC-Run-&lt;version&gt;.tar.gz</filename> source archive from CPAN,
+      <filename>IPC-Run-&lt;version&gt;.tar.gz</filename> source archive from
+      <acronym>CPAN</acronym>,
       at <ulink url="https://metacpan.org/dist/IPC-Run"></ulink>, and
       uncompress. Edit the <filename>buildenv.pl</filename> file, and add a PERL5LIB
       variable to point to the <filename>lib</filename> subdirectory from the
@@ -518,6 +519,11 @@ $ENV{PROVE_TESTS}='t/020*.pl t/010*.pl'
 </programlisting>
   </para>
 
+  <para>
+    Additionally, the behavior of TAP tests can be controlled by a set of
+    environment variables, see <xref linkend="regress-tap-vars" />.
+  </para>
+
   <para>
    Some of the TAP tests depend on a set of external commands that would
    optionally trigger tests related to them. Each one of those variables

doc/src/sgml/regress.sgml

@@ -765,7 +765,9 @@ make check PROVE_TESTS='t/001_test1.pl t/003_test3.pl'
 
    <para>
     The TAP tests require the Perl module <literal>IPC::Run</literal>.
-    This module is available from CPAN or an operating system package.
+    This module is available from
+    <ulink url="https://metacpan.org/dist/IPC-Run">CPAN</ulink>
+    or an operating system package.
     They also require <productname>PostgreSQL</productname> to be
     configured with the option <option>--enable-tap-tests</option>.
    </para>
@@ -789,6 +791,30 @@ make check PROVE_TESTS='t/001_test1.pl t/003_test3.pl'
     meaning that <literal>make installcheck</literal> will produce a mix of
     results from temporary servers and the already-running test server.
    </para>
+
+  <sect2 id="regress-tap-vars">
+   <title>Environment variables</title>
+
+   <para>
+    Data directories are named according to the test filename, and will be
+    retained if a test fails.  If the environment variable
+    <varname>PG_TEST_NOCLEAN</varname> is set, data directories will be
+    retained regardless of test status.  For example, retaining the data
+    directory regardless of test results when running the
+    <application>pg_dump</application> tests:
+<programlisting>
+PG_TEST_NOCLEAN=1 make -C src/bin/pg_dump check
+</programlisting>
+   </para>
+
+   <para>
+    Many operations in the test suites use a 180-second timeout, which on slow
+    hosts may lead to load-induced timeouts.  Setting the environment variable
+    <varname>PG_TEST_TIMEOUT_DEFAULT</varname> to a higher number will change
+    the default to avoid this.
+   </para>
+  </sect2>
+
   </sect1>
 
   <sect1 id="regress-coverage">