tests: list of tests placed in a separate Makefile fragment

tests: list of tests placed in a separate Makefile fragment

This change allows us to easily use make instead of by-hand (and
more fragile) grepping to extract the list of hand-written tests
from our Makefiles.

* bootstrap (BOOTSTRAP_MAKE): New variable.
Use it to run make in order to extract the list of hand-written
tests from ...
* tests/list-of-tests.mk: ... this new file, where they are
listed in ...
(handwritten_TESTS, tap_with_common_setup_TESTS, tap_other_TESTS):
... these variables, extracted respectively from ...
* tests/Makefile.am (handwritten_tests, tap_other_tests,
tap_with_common_setup_tests): ... these older variables, now
removed.
(EXTRA_DIST, expected_list_of_tests): Update w.r.t. the variable
names' changes.
(include $(srcdir)/list-of-tests.mk): New inclusion.
Remove an overkill indirection, and with it ...
(tap_with_common_setup_logs): ... this variable.
($(srcdir)/parallel-tests.am, $(srcdir)/config-shell-tests.am):
Update to match the new interface of ...
(gen-parallel-tests, gen-config-shell-tests): ... these scripts,
that now read the list of tests from the standard input.