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

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.