test defs: move in 't/ax/test-defs.sh'

test defs: move in 't/ax/test-defs.sh'

We can do so with minimal churn, now that the early setup of $PATH in
both 'runtest' and AM_TESTS_ENVIRONMENT allow the '.' built-in to find
to-be-sourced shell scripts in 't/ax' automatically, both in in-tree
and  VPATH builds.

With this change, we take another step forward a more rational and
"segregated" organization of our testsuite framework.

* defs-static.in: Rename ...
* t/ax/test-defs.in: ... like this, and adjust.
* Makefile.am, syntax-checks.mk, t/README, gen-testsuite-part,
t/tests-init.sh, t/ax/tap-setup.sh, t/tap-summary.sh, .gitignore,
t/testsuite-summary-color.sh, t/testsuite-summary-count.sh,
t/self-check-tap.sh, defs: Adjust.

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