coverage: test that AM_PROG_MKDIR_P and $(mkdir_p) still works

coverage: test that AM_PROG_MKDIR_P and $(mkdir_p) still works

They are deprecated, but should continue to work in the 1.12.x
release series.  Report from Benoit Sigoure and Diego Elio Pattenò:
<http://lists.gnu.org/archive/html/automake/2012-07/msg00001.html>

* t/mkdirp-deprecation.sh: Enhance.
* t/mkdir_p.sh: New test, check that AM_INIT_AUTOMAKE still defines
the $(mkdir_p) make variable.  Currently xfailing.
* t/list-of-tests.mk (handwritten_TESTS, XFAIL_TESTS): Add the new
test.

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