tests: remove t/parallel-tests-many.sh (fixes spurious failure)

tests: remove t/parallel-tests-many.sh (fixes spurious failure)

That test was checking for automake#7868 "parallel-tests and command-line
length limit issue".  That bug is still open, and the tests was xfailing.

What concerns us is that the test was also causing an annoying problem.
On Solaris 10, it produced a '.log' file with two overly-long lines (more
than 1 million characters long!) and that was causing the test harness to
experience spurious failures, with Solaris XPG4 awk complaining that:

 /usr/xpg4/bin/awk: line 0 (NR=734): Record too long (LIMIT: 19999 bytes)

A little consideration shows that the bug#7868 checked by the culprit
test cannot be solved "by accident", e.g., by another bug fix or some
simple refactoring, so there is little added value in keeping the xfailing
test for it, if this causes other problems (and we've seen it does).

So we just remove the test.  In case we ever manage to fix the bug in
mainline automake, we can copy over the similar test(s) from Automake-NG,
which has actually managed to fix the bug (and have simpler test cases
for it).

* t/parallel-tests-many.sh: Delete.
* t/list-of-tests.mk (XFAIL_TESTS, handwritten_TESTS): Remove it.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>