[ng] refactor: merge check2.am into parallel-tests.am

[ng] refactor: merge check2.am into parallel-tests.am

Another change aimed at moving logic and processing out of automake
and inside the generated Makefiles.  This is mostly a preparatory
step.

* lib/am/check2.am: Merged ...
* lib/am/parallel-tests.am: ... in here, with the (several) required
adjustments and extensions.
* Makefile.am (dist_am_DATA): Adjust.
(XFAIL_TESTS): Add 't/test-extensions.sh'.
* automake.in (handle_per_suffix_test): Don't process nor include
'check2.am' anymore.
* t/parallel-tests-internals.sh: New test.
* t/serial-tests.sh: Adjust grepping checks.

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