tests: expose automake bug#14560

tests: expose automake bug#14560

Automake needlessly generates definition of make variables related
to C compilation when two or more user-defined suffix rules are
present in a single Makefile.am.

* t/suffix-extra-c-stuff-pr14560.sh: New test, exposing the bug.
This test is still xfailing.
* t/no-extra-c-stuff.sh: New test, check for a potential related
regression.  This regression is not actually present here (so this
test passes), but it still took place in our first attempt at
fixing bug#14560 -- so this test has proven to be actually useful.
* t/no-extra-makefile-code.sh: Improve comments, and tighten the
grepping checks a little.
* t/list-of-tests.mk (handwritten_TESTS): Add the new tests.
(XFAIL_TESTS): Add the new xfailing test.

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