testsuite: more environment sanitization

testsuite: more environment sanitization

* tests/defs: Move the checks verifying that the variables `me',
`parallel_tests' and `required' aren't set in the environment ...
* tests/defs-static.in: ... in here, with some optimizations to
avoid useless forks.  Also, do the same checks for the variables
`original_AUTOMAKE' and `original_ACLOCAL' too.
* tests/self-check-env-sanitize.test: Update.
* tests/Makefile.am (TESTS_ENVIRONMENT): Unset also variables
`original_AUTOMAKE' and `original_ACLOCAL'.