testsuite: introduce --release

* tests/atlocal.in, tests/local.at: Support option --release, which sets
BISON_USE_PUSH_FOR_PULL and BISON_TEST_XML.
* tests/local.mk: Use it for maintainer-release-check.

tests/local.at

@@ -1022,6 +1022,9 @@ m4_define([AT_SETUP],
 AT_ARG_OPTION([compile-c-with-cxx],
               [compile C parsers with the C++ compiler])
 
+AT_ARG_OPTION([release],
+              [check xml and use push for pull])
+
 AT_COLOR_TESTS
 
 AT_TESTED([bison])