tests: extend tests on dependency tracking with VPATH

tests: extend tests on dependency tracking with VPATH

* tests/depmod-tests.sh: New script, fulfilling a threefold role:
1. it is called to generate a Makefile.am snippet, containing the
   definition of proper lists of tests;
2. it is called to set up a directory containing some common data
   files and autotools-generated files used by the aforementioned
   tests (this is done for speed reasons only); and
3. it is called to properly run those tests, one at a time.
* tests/depcomp9.test: Delete, it's obsolete now.
* tests/depcomp10.test: Likewise.
* tests/Makefile.am ($(srcdir)/depmod-tests.am): Include this
snippet, which defines ...
(depmode_tests): ... this new macro, containing the list of the
newly generated `*.depmod' tests.
(TESTS_EXTENSIONS): Add `.depmod'.
(DEPMOD_LOG_COMPILER): Define.  It calls `depmod-tests.sh', so that
the "depmode tests" will be executed by passing that driver script
a proper parameter.
($(depmod_tests)): New dependency declaration (dummy, but required
in order to have make actually produce expected log files from the
`.depmod.log' suffix rule).
(TESTS): Add $(depmod_tests).
(EXTRA_DIST): Distribute depmod-tests.sh.
Other minor cosmetic changes and reorderings.
* bootstrap: Generate depmod-tests.am.
* tests/.gitignore: Updated.