[ng] build: define $(TESTS) through a wildcard

[ng] build: define $(TESTS) through a wildcard

We can finally do that thanks to our recent improvements to the parallel
testsuite harness :-)

* t/list-of-tests.mk: Delete.
* t/CheckListOfTests.am: Likewise, the checks it implemented being now
redundant.
* syntax-checks.mk (maintainer-check): Don't depend on the obsolete and
now removed 'maintainer-check-list-of-tests' check anymore.
* Makefile.am: Don't include those deleted makefile fragments anymore.
(TESTS): Define through a proper use of the $(wildcard) GNU make builtin.
(perl_fake_XFAIL_TESTS, XFAIL_TESTS): Moved in from 't/list-of-tests.mk'.
($(srcdir)/t/testsuite-part.am): Don't generate nor include anymore, ...
($(srcdir)/autodeps.am): ... generate and include this instead.
Remove explicit dependency of the generated tests and their list on the
'$(srcdir)/gen-testsuite-part' file; while technically correct, that was
overly picky and source of problems for casual testers/users.  And anyway,
we don't have an explicit list of all the generated files anymore ...
* bootstrap: Adjust, by generating 'testsuite-autodeps.am' rather than
't/testsuite-part.am'.
* .gitignore: Update.
* t/self-check-dir.tap: Update to avoid a spurious failure due to the
changes in Makefile.am.
* gen-testsuite-part: Do not output the list of automatically generated
tests.  Ensure the generated tests are always placed in the $(srcdir).
Assume (here and in the generated scripts) that all the test scripts
(whther autogenerated or not) are to be found in $(srcdir).  Adjust some
comments.

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