tests init: don't bother allowing '$me' to be overridable

tests init: don't bother allowing '$me' to be overridable

We once used that feature in our wrapper tests; but now (and probably
even since commit 'v1.11-1308-g375f23d' of 2011-09-08, "testsuite:
revamp generation of autogenerated tests") it is not needed anymore.
By removing it we can simplify our growingly complex testsuite framework
a little.

* t/ax/test-inist.sh ($me): Do not initialize it here (and only if not
already set), instead ...
* defs-static.in ($me): ... initialize it here unconditionally.
Do not check anymore that $me doesn't come from the environment: that
wouldn't cause any problem now.
Now that '$me' is defined early, prefer it over 'argv0' in early error
messages, both here ...
* defs: ... and here.
* Makefile.am (AM_TESTS_ENVIRONMENT): Do not bother "nullifying" $me
anymore.
* t/self-check-env-sanitize.tap: Adjust.
* t/self-check-me.tap: Likewise.

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