[ng] coverage: parallel-tests and dynamic $(TESTS) content

[ng] coverage: parallel-tests and dynamic $(TESTS) content

The recent changes to the parallel-tests implementation should allow us
to define or override TESTS with more dynamic contents; in particular,
something like:

  TESTS = $(wildcard t[0-9][0-9]*.sh)

should now work smoothly.  Let's cover this enhanced functionalities
explicitly in our testsuite.

* t/parallel-tests-dynamic.sh: New test.
* t/list-of-tests.mk: Add it.
* NG-NEWS: Document the new feature.

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