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.