[ng] parallel-tests: do not exceed command line length limits

[ng] parallel-tests: do not exceed command line length limits

Fixes automake bug#7868.

Two things worth noting:

 + a "make recheck" issued after a huge number of tests have failed
   can still hit command-line length issues;

 + the check-recipes now contain (first among the Automake-generated
   recipes) a use of the 'xargs' utility.

These issues will likely be tackled by later patches.

* Makefile.am (XFAIL_TESTS): Remove 't/parallel-tests-many.sh'.
* lib/am/clean.am (.am.clean-cmd.f, .am.clean-cmd.d, .am.rm-f,
.am.rm-rf): Move their definitions ...
* lib/am/header-vars.am: ... here, because we need to use them ...
* lib/am/parallel-tests.am: ... here as well.
(am.test-harness.workdir, am.setup-test-harness-workdir,
am.test-harness.append-to-list-of-bases): New internal variables.
Use them to avoid hitting command-line length limits ...
($(TEST_SUITE_LOG), recheck): ... in this rules ...
(am__remove_if_not_lazy_check): ... and in the shell code defined
by this internal variable.

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