Allow use of SB-RT's mechanism for expected test failures in contribs

Allow use of SB-RT's mechanism for expected test failures in contribs

... In RT, use the list of expected test failures not just for
    debugging output, but return as a primary result only whether
    there are unexpected failures, with additional result values for
    details on expected failures.

... Tweak make-target-contrib.sh so that the `test-passed' file, if
    non-empty, can indicate a `successful' build of the contrib with
    only known failures.

This mechanism is meant to follow in the footsteps of SBCL's main
test suite's mechanism for known failures, and is arguably not in
the original spirit of contribs as user contributions that must
build perfectly or fail to install entirely.  However, for parts of
contrib/ which are very commonly used as a part of SBCL, it is hoped
that this new mechanism will aid work toward bug fixes in those
contribs rather than de-emphasize it.