Revamp test-related Makefiles.

Revamp test-related Makefiles.

One side-effect of this change is that "make check" now works even if
you put "." early in your shell's search PATH (don't do that!).

Remove all test-related Makefile.am files, except those generated
by mk-script.  Instead, tests/Makefile.am now lists not only the
tests directly under tests/, but also those in tests/*/ that are
not generated by mk-script, e.g., cp/abuse, cp/acl, mv/i-1, etc.

A lot of these changes are like this:

-. $srcdir/../lang-default
+. $top_srcdir/tests/lang-default

-. $srcdir/../test-lib.sh
+. $top_srcdir/tests/test-lib.sh

* configure.ac (AC_CONFIG_FILES): Remove corresponding Makefiles.
* tests/check.mk (vc_exe_in_TESTS): Relax syntax requirements.
* tests/rwx-to-mode: Remove file.  Rewritten as...
* tests/test-lib.sh (rwx_to_mode_): ...this new function.
* tests/Makefile.am (EXTRA_DIST): Remove rwx-to-mode.
(SUBDIRS): Remove each dir with a removed Makefile.am.
(EXTRA_DIST): Add $(TESTS).
(TESTS): Add over 300 entries.