testsuite: more environment sanitization

testsuite: more environment sanitization

* tests/defs.in: Sanity check: abort if any of `parallel_tests'
or `required' is in the environment.
($sed_unindent_prog): Initialize to empty, to avoid interferences
from the environment.
* tests/self-check-me-in-env.test: Renamed to ...
* tests/self-check-env-sanitize.test: ... this, and extended.
* tests/Makefile.am (TESTS): Update.
(TESTS_ENVIRONMENT): Unset variables `parallel_tests' and
`required'.  Adjust comments.