test setup: merge definitions of function for simple tests

test setup: merge definitions of function for simple tests

* t/ax/plain-functions.sh: Delete, moving the definitions of ...
($stderr_fileno_): ... this variable ...
(warn_, fail_, skip_, skip_all_, fatal_, framework_failure_): ... and
these functions ...
* t/ax/test-init.sh: ... in here.  This allow us to use those functions
earlier in this file (instead of having to duplicate their behaviour
with inlined code).  The TAP-based tests are still able to override these
functions later to their TAP-enhanced equivalents when 'tap-functions.sh'
is sourced.
* Makefile.am (dist_noinst_DATA): Remove 't/ax/plain-functions.sh'.
* t/ax/tap-functions.sh: Adjust a comment.

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