self tests: some new uses of TAP, few extensions

self tests: some new uses of TAP, few extensions

* tests/self-check-me.test: Renamed ...
* tests/self-check-me.tap: .. to this, and converted to use TAP.
* tests/self-check-exit.test: Renamed ...
* tests/self-check-exit.tap: .. to this, and converted to use TAP.
Set `$stderr_fileno_' to `2', to avoid spurious messages on the
console.  Add a new test, and relax another one.
* tests/self-check-env-sanitize.test: Renamed ...
* tests/self-check-env-sanitize.tap: .. to this, and converted to
the use TAP.
* tests/Makefile.am (TESTS): Update.