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/atlocal.in

@@ -75,6 +75,15 @@ else
   CC_IS_CXX=0
 fi
 
+# In a similar way, handle --release
+if "$at_arg_release"; then
+  BISON_USE_PUSH_FOR_PULL=1
+  BISON_TEST_XML=1
+else
+  BISON_USE_PUSH_FOR_PULL=0
+  BISON_TEST_XML=0
+fi
+
 
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 ## Other.  ##