parallel-tests: don't allow @substitutions@ in TEST_EXTENSIONS

parallel-tests: don't allow @substitutions@ in TEST_EXTENSIONS

Even after the last commit `v1.11-476-g90bea64', the checks
performed by automake on entries in $(TEST_EXTENSIONS) tried to
allow for @substitited@ stuff.  This however ends up allowing
quite brittle setups, which, most importantly, are of no real
practical usefulness anyway.  So it's better to just disallow
@substitutions@ in TEST_EXTENSIONS altogether, offering a clear
error message, instead of risking weird bugs and unexpected
behaviors in the generated Makefile.in.

* automake.in ($TEST_EXTENSION_PATTERN): Turned from a regular
expression ...
(is_valid_test_extension): ... into this subroutine.  Don't allow
generic @substitutions@ anymore (possibly making an exception for
`@EXEEXT@' under the proper circumstances).
* tests/test-extensions.test: Adjust and extend.