tests: "am_using_tap=yes" -> "am_test_protocol=tap"

tests: "am_using_tap=yes" -> "am_test_protocol=tap"

And similarly, "am_using_tap=no" -> "am_test_protocol=none".

The new '$am_test_protocol' name is clearer, and will allow the easy
addition of further test protocols in the future.  This is not truly
relevant for automake, but we are trying to make some parts of our
testsuite framework as general as possible, in view of a future move
to a more generic project like Gnulib.

* Makefile.am, t/ax/am-test-lib.sh, t/ax/test-defs.in, t/ax/test-lib.sh,
t/self-check-env-sanitize.tap, t/self-check-tap.sh: Adjust.
* syntax-checks.mk (sc_tests_obsolete_variables): Add 'am_using_tap' to
the list of obsolete variables to check against.

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