When debugging these parsers, we really need debug traces.
Enable them, and bind them to $YYDEBUG.
* tests/glr-regression.at: Support the YYDEBUG envvar.
As a consequence, now that syntactic ambiguities are reported, adjust
the expected output.
(No users destructors if stack 0 deleted): Don't return 0 on memory
exhaustion, really return the parser's status, and adust expectations.