maintcheck: consistency of list of test scripts

maintcheck: consistency of list of test scripts

This is basically a backport of commit 'v1.11-358-g7b6ab07'
and its follow-ups.  The possibility of easily checking that
the list of test scripts listed in a Makefile equals that of
the tests on the filesystem has proved itself so useful that
it's worth taking the annoyance of backporting it to maint.

* CheckListOfTests.am: New file, backported from master (commit
'v1.11-1736-g083a75b') with minor adjustments.
(maintainer-check-list-of-tests): New target, check for consistency
between the list of tests defined in the including Makefile and the
list of tests on the filesystem.
(clean-maintcheck-testslist-tmp): New rule, to clean up temporary
files that might be left around by the rules associated with the
previous target.
(clean-local): Depend on it.
* lib/Automake/tests/Makefile.am: Include `CheckListOfTests.am'.
* tests/Makefile.am: Likewise.
* Makefile.am (maintainer-check-list-of-test): New target,
calling recursively into `tests/' and `lib/Automake/tests/',
using ...
(TEST_SUBDIRS): ... this new variable.
(maintainer-check): Added dependency from the new target
`maintainer-check-list-of-tests'.