maint: fix syntax-check issues

* cfg.mk: Ignore strcmp in local.at.
* tests/conflicts.at: Use AT_PARSER_CHECK.
* tests/regression.at: Preserve the exit status of the generated parsers.

* tests/local.mk ($(TESTSUITE)): Map @tb@ to a tabulation.
* tests/c++.at, tests/input.at, tests/regression.at: Use @tb@.
* cfg.mk: (space-tab): There are no longer exceptions.

tests/conflicts.at

@@ -80,7 +80,7 @@ int main (void)
 
 AT_FULL_COMPILE([input])
 
-AT_CHECK([./input])
+AT_PARSER_CHECK([./input])
 
 AT_BISON_OPTION_POPDEFS