maintcheck: fix spurious failures

maintcheck: fix spurious failures

* tests/gen-testsuite-part (write_wrapper_script): More uses of
creative quoting in the generated tests, to avoid triggering
failures in the `sc_tests_Exit_not_exit' maintcheck.
* Makefile.am (sc_tests_tap_plan): It's OK for auto-generated
TAP tests that sources an hand-written one not to declare a TAP
plan: the sourced test will do so.
* tests/Makefile.am (checked_test_extensions): Don't define
anymore, the default value `$(TEST_EXTENSIONS)' is correct now.
(expected_list_of_tests): Don't define anymore, the default
value `$(TESTS)' is correct now.