Clean up mono/tests/Makefile.am.

This removes a lot of duplicated logic and special-casing for Wrench. There
are now only two important top-level targets:

* `check`: Run all stable (~fast) tests.
* `check-full`: Run all stable, unstable, and slow tests.

By default, `no-dump` is passed to the test runner. To pass `dump-output` and
get `stdout`/`stderr` dumped to console when a failure occurs, set `V` as in:

    $ make check V=1

Or:

    $ make check-full V=1
Ignore testlist files in mono/tests.