tests: add a syntax check for last week's global change

Last week I made a global change, commit a2c811db, `tests: use
"compare exp out", not "compare out exp"', but forgot to add a
corresponding syntax check rule.  Without that, it is far too
easy to add a new test or to merge in an old one that would
be non-conforming.  Obviously this is only a heuristic, since
it relies on the expected-output file to have a name that starts
with "exp".
* cfg.mk (sc_prohibit_reversed_compare_failure): Prohibit use of
compare with reversed arguments.

cfg.mk

@@ -280,6 +280,12 @@ sc_prohibit_framework_failure:
 	halt='use framework_failure_ instead'				\
 	  $(_sc_search_regexp)
 
+# In tests, use "compare expected actual", not the reverse.
+sc_prohibit_reversed_compare_failure:
+	@prohibit='\<compare [^ ]+ (exp|/dev/null)'			\
+	halt='reversed compare arguments'				\
+	  $(_sc_search_regexp)
+
 ###########################################################
 _p0 = \([^"'/]\|"\([^\"]\|[\].\)*"\|'\([^\']\|[\].\)*'
 _pre = $(_p0)\|[/][^"'/*]\|[/]"\([^\"]\|[\].\)*"\|[/]'\([^\']\|[\].\)*'\)*