doc: mention libpq regression tests

Reported-by: Ryo Matsumura

Discussion: https://postgr.es/m/TYCPR01MB11316B3FB56EE54D70BF0CEF6E8E4A@TYCPR01MB11316.jpnprd01.prod.outlook.com

Backpatch-through: 11

doc/src/sgml/regress.sgml

@@ -196,8 +196,9 @@ make check-world -j8 >/dev/null
    </listitem>
    <listitem>
     <para>
-     Regression tests for the ECPG interface library,
+     Regression tests for the interface libraries,
-     located in <filename>src/interfaces/ecpg/test</filename>.
+     located in <filename>src/interfaces/libpq/test</filename> and
+     <filename>src/interfaces/ecpg/test</filename>.
     </para>
    </listitem>
    <listitem>