check: separate .log -> .html conversion from core testsuite harness

check: separate .log -> .html conversion from core testsuite harness

That feature is to be deprecated in the 1.11.x series, and removed
from the automake core in the 1.12 release, where it will instead
be offered in a semi-independent extra '*.am' fragment.  So let's
start better separating the .log -> .html conversion from the "core
code" of the parallel-tests harness.

Reference:
  <http://lists.gnu.org/archive/html/automake/2012-01/msg00005.html>

* lib/am/check.am (.log.html, check-html, recheck-html): Move these
targets ...
* automake.in (handle_tests): ... and the initialization of the
TEST_SUITE_HTML variable and the cleaning of the $(TEST_SUITE_HTML)
file ...
* lib/am/check-html.am: ... in this new file, with related (minor)
refactorings, enhancements and simplifications.
* lib/am/check.am (.MAKE. PHONY, AM_RECURSIVE_TARGETS): Adjust.
* lib/am/Makefile.am (dist_am_DATA): Add 'check-html.am'.
* tests/parallel-tests2.test: Improve coverage.