tests: move 'runtest' into the top-level directory

tests: move 'runtest' into the top-level directory

Move our wrapper script 'runtest' (meant to allow the execution of
Automake test cases from the command line) from the 't/ax/' directory
to the top-level one.  This makes the script easier to find and to
invoke.  Much more importantly, our DejaGNU-checking test cases won't
try anymore to use that script instead of the 'runtest' program provided
by DejaGNU (that happened because '$(srcdir)/t/ax/' is automatically
added early to the $PATH variable in our test cases), which was causing
spurious SKIPs.

* t/ax/runtest.in: Move ...
* runtest.in: ... here.
* Makefile.am, t/README, .gitignore: Adjust.

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