tests defs: more namespace-safeness

tests defs: more namespace-safeness

($using_tap): Renamed ...
($am_using_tap): ... to this.
($parallel_tests): Renamed ...
($am_parallel_tests): ... to this.
($test_prefer_config_shell): Renamed ...
($am_test_prefer_config_shell): ... to this.
($original_AUTOMAKE): Renamed ...
($am_original_AUTOMAKE): ... to this.
($original_ACLOCAL): Renamed ...
($am_original_ACLOCAL): ... to this.
* tests/defs, tests/defs-static.in, tests/gen-testsuite-part,
tests/README, and all callers: Adjusted.
* tests/Makefile.am (AM_TESTS_ENVIRONMENT): Adjust, and rewrite
to make future adjustments easier.
* tests/defs (funcs_file_): Renamed ...
(am_funcs_file): ... to this.
* Makefile.am (sc_tests_obsolete_variables): New maintainer
check.
(syntax_check_rules): Add it.