maintcheck: completeness check on list of tests simplified

maintcheck: completeness check on list of tests simplified

* CheckListOfTests.am: Simplify the recipe, accordingly to the
assumption that all the entries in $(TEST_EXTENSIONS) and in
$(TESTS) have an associated real test script.  This was not the
case when we used to have some entries in $(TESTS) that were not
real files, but rather "parameters" (sort of) for the corresponding
$(LOG_COMPILER).  However, that usage had proven to be confusing
and brittle, and thus removed (see commit 'v1.11-1308-g375f23d' of
September 8 2011, "testsuite: revamp generation of autogenerated
tests").