maint: avoid parsing of Makefile.am from vc_exe_in_TESTS

maint: avoid parsing of Makefile.am from vc_exe_in_TESTS

* tests/Makefile.am (TESTS): Rename ...
(all_tests): ... like this, so that we'll still be able to know the
complete list of our tests even if the user overrides TESTS from the
command line (which he's allowed to do by the test harness API).
(root_tests): Rename ...
(all_root_tests): ... like this, for similar reasons.
(TESTS, root_tests): Redefine their defaults to to $(all_tests) and
$(all_root_tests) respectively.
(vc_exe_in_TESTS): It can now safely use $(all_tests) to get the
complete list of test cases according to the Makefile, instead of
having to resort to "parsing" of Makefile.am.
(EXTRA_DIST): Use $(all_tests), not $(TESTS).
(v_, w_): Delete, no longer needed.