tests: some minor fixlets w.r.t. TAP-based tests

tests: some minor fixlets w.r.t. TAP-based tests

The variable that is used by our test scripts to decide whether
they are "plain" or TAP-based is `using_tap'; however, some
peripheral sanity checks and sanitization code in our testsuite
were erroneously using the `use_tap' variable instead.  Fix that.

* tests/Makefile.am (AM_TESTS_ENVIRONMENT): Nullify `using_tap',
not `use_tap'.
* tests/defs-static.in: Check that `using_tap', not `use_tap',
is not exported.
* tests/self-check-env-sanitize.test: Update.