[ng] check: per-suffix dependencies for test cases

[ng] check: per-suffix dependencies for test cases

Akim Demaille asked (in automake bug#11287) for a way to specify extra
dependencies for the test cases, depending on their extension (or lack
thereof).  Now that Automake-NG uses pattern rules instead of old-fashion
suffix rules to generate '.log' and '.trs' files from test cases, doing
that is trivial.

* NG-NEWS: Update.
* doc/automake.texi: Document the new feature.
* automake.in (handle_per_suffix_test): Mark the contents of the
'${pfx}LOG_DEPENDENCIES' variable as processed (required to avoid
spurious errors).
* lib/am/check2.am (%.log, %.trs): Depend on '$(%PFX%LOG_DEPENDENCIES)'.
* t/parallel-tests-per-suffix-deps.sh: New test.
* t/parallel-tests-log-compiler-example.sh: Renamed ...
* t/parallel-tests-logvars-example.sh: ... to this, and adjusted to
be more faithful to the updated documentation.

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