[ng] deptrack: increase testsuite coverage

[ng] deptrack: increase testsuite coverage

This will be useful in the soon-to-appear refactoring to the code
supporting automatic dependency tracking.

* tests/depcomp-recover.test: New test, still xfailing.
* tests/Makefile.am (XFAIL_TESTS): Add it.
* tests/depcomp-shuffle.sh: New helper script, sourced by ...
* tests/depcomp-shuffle-so.test, tests/depcomp-shuffle-vpath.test,
* tests/depcomp-shuffle.test, tests/depcomp-shuffle-so-vpath.test:
... these new tests.
* tests/list-of-tests.mk: Add all the new tests.
* tests/gen-testsuite-part ($deps_extractor[depcomp_shuffle]): Also
compute dependencies for the new tests.