Regarding these ASDF failures on MKCL:
* test-program was a bug in my latest ASDF tweaks, now fixes.
* test-bundle and test3 smell like another case where MKCL errors out
due to non-serious conditions being signalled, this time inside a
require.

* test-defsystem-depends-on-phase-overlap.script is because said test
tries to undefine a class using
(setf (find-class 'asdf::my-cl-source-file) nil)
and MKCL disapproves, unlike all other tested CL implementations, erroring with:
"TEST ABORTED: NIL is not of type SI:INSTANCE."
Is that a compliant way to undefine a class? If not, is there one and
what is it?