tests: prepare to move ./defs to t/ax/test-init.sh

tests: prepare to move ./defs to t/ax/test-init.sh

We don't do this in a sweeping passage, because that would cause
endless headaches in the synchronization between the maint, master
and ng/master branches.  Instead, we setup our framework to allow
test scripts to work by sourcing either './defs' or 'test-init.sh',
so that we'll be able to make the transition gradual and painless.

* t/ax/test-init.sh: New, copied from the previous ./defs file.
* defs: Simply work by sourcing the new file.
* Makefile.am (dist_noinst_DATA): List the new file.
* t/README: Adjust to mandate the sourcing of 'test-init.sh' rather
than of './defs'.
* t/c-demo.sh: Source 'test-init.sh' instead of ./defs.  This is
done to verify our new setup actually works.
* t/ac-output-old.tap: Likewise.

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