[ng] parallel-tests: reduce code duplication a little

[ng] parallel-tests: reduce code duplication a little

Use more GNU make features and built-ins, and less automake-time
pre-processing, in order to reduce code duplication and improve
clarity.

* lib/am/check2.am: Define a new custom make function 'am__runtest', used
to simplify rules in here, and reduce code duplication among them.
* automake.in (handle_per_suffix_test): Update and simplify accordingly.
As a result, the automake-defined 'LOG_COMPILE' variables are not defined
anymore, which we should mention ...
* NG-NEWS: ... in here.
* t/test-extensions.sh: Adjust outdated grepping checks.

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