runtest: sanitize test environment

runtest: sanitize test environment

* runtest.in: Here, similarly to what is done by AM_TESTS_ENVIRONMENT
in Makefile.am, unset variables that should be under the complete control
of the test framework, and that could create havoc if inherited from the
environment.  This remove the need to check against possible environment
"pollution" ...
* t/ax/test-defs.in: ... in here.
* Makefile.am (AM_TESTS_ENVIRONMENT): Add a comment about the need of
synchronization with 'runtest.in'.
* t/self-check-env-sanitize.tap: Remove as obsolete.
* t/list-of-tests.mk: Adjust.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>