[ng] contrib: rewrite and fix 'check-html' and 'recheck-html'

[ng] contrib: rewrite and fix 'check-html' and 'recheck-html'

That has actually been broken for some time, due to the changes
done in Automake-NG proper in the meantime.

* contrib/check-html.am (.log.html): Re-declare this suffix rule ...
(%.html: %.log): ... as a pattern rule.
(mostlyclean-check-html): Remove, instead ...
(MOSTLYCLEANFILES): ... appending to this variable.  Not only this
reduce the code size, but will give us resilience against possible
exceeded command line length limits for free.
(check-html, recheck-html): Rewrite as static pattern rules.  Fix
the recursive "make check" and "make recheck" invocation to override
TESTS, not TEST_LOGS (the latter override is not honoured anymore
by the new incarnation of the parallel-test rules).
* contrib/t/parallel-tests-html-recursive.sh: Adjust a little, by
initializing 'MOSTLYCLEANFILES' (to empty) in all the 'Makefile.am'
files that use 'check-html.am'.
* contrib/t/parallel-tests-html.sh: Likewise.  Also, since the
parallel-tests API does not allow command-line overriding of
TEST_LOGS anymore, stop doing that.

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