tests: move all under the same hierarchy ('tests/' directory)

tests: move all under the same hierarchy ('tests/' directory)

This simplifies the organization of the Automake source tree and
reduces the (lamentably high) number of Makefiles in the Automake
build system by one.  It also makes the maintainer check that
verifies the consistency of list of tests more self-contained and
simpler.  Finally, it might be a first step forward the transition
to a non-recursive build system for automake (if we ever decide to
go down that road fully some day).

* lib/Automake/tests: All the '*.pl' tests in here moved ...
* tests/pm: ... into this new directory.
* lib/Automake/tests/Makefile.am: Remove, its meaningful
contents moved ...
* tests/Makefile.am: ... here, with obvious adjustments.
(test_subdirs): New variable, for the sake of the recipe
of 'maintainer-check-list-of-tests'.
* CheckListOfTests (maintainer-check-list-of-tests): Enhance
its recipe to make it able to deal with test script residing
in subdirectories.
* Makefile.am (maintainer-check-list-of-tests): Simplified.
(TEST_SUBDIRS): Remove, no more needed.
* tests/list-of-tests.mk (perl_TESTS): New variable, lists
the '.pl' tests just moved into 'tests/pm'.
(handwritten_TESTS): Add the contents of '$(perl_TESTS)'.
* lib/Automake/Makefile.am (SUBDIRS): Remove.
* configure.ac (AC_CONFIG_FILES): Update.
* .gitignore: Adjust.