coverage: expose automake bug#7868

coverage: expose automake bug#7868

Expose the command-line length limit issue that can affect the
Automake-generated parallel-tests harness, especially on systems
where this limit is smaller (e.g., MinGW/MSYS).

Suggestion by Bob Friesenhahn.

* tests/parallel-tests-many.test: New test.  We have verified that
it actually exposes the bug#7868, as it passes when we opportunely
reduce the number of test cases in $(TESTS).  Checked on NetBSD 5.1
64bit, Debian unstable 32bit, Solaris 10 64bit and Cygwin 1.5 32bit.
* tests/list-of-tests.mk (handwritten_TESTS): Add it.
* tests/Makefile.am (XFAIL_TESTS): Likewise.