[ng] recheck: improve the fix for bug#11791 to use more GNU make features

[ng] recheck: improve the fix for bug#11791 to use more GNU make features

* lib/am/parallel-tests.am: Here, and to generally have a "look and feel"
more in the Automake-NG style.  Also, improve comments while we are at it.
* Makefile.am (XFAIL_TESTS): Remove 't/test-missing.sh', as the changes in
'parallel-tests.am' have restored some semantics that had been messed up
in the previous merge (where 't/test-missing.sh' started failing).

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