spy: "rm -f" and "rm -rf" without args are portable

According to recent testing by Eric Blake, it seems that all
non-museum rm(1) implementations don't complain if called without
file operands, *if* the '-f' option is used.  This behaviour will
likely be mandated by future versions of the POSIX standard as
well.

We'd like to start assuming this rm(1) behaviour in future Automake
releases, in order to simplify some automake-provided cleanup rules.
So, for the moment, we add a new "spy" test in our test suite, to
try ensuring that all the rm(1) implementation we encounter "in the
wild" truly behaves as expected in this regard.

For more references, see this discussion on the bug-autoconf list:
 <http://lists.gnu.org/archive/html/bug-autoconf/2012-02/msg00002.html>
which sprang from coreutils bug#10819:
 <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=10819>
and brought to the POSIX issue#542:
 <http://austingroupbugs.net/view.php?id=542>
which has been accepted and will ensure that future version of the
POSIX standards will mandate that 'rm' shouldn't complain if called
without file operands, when the '-f' option is in use.

* tests/spy-rm.test: New test.
* tests/list-of-tests.mk: Add it.

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