test: fix test syntax for ASDFv3

[hraban]

Fix tests on ASDF v3. Fixes this error:

CL-USER> (ql:quickload "cl-difflib-tests")
CL-USER> (asdf:test-system :cl-difflib)
:FORCE and :FORCE-NOT arguments not allowed in a nested call to ASDF/OPERATE:OPERATE unless identically to toplevel
   [Condition of type UIOP/UTILITY:PARAMETER-ERROR]

Looping in @fare : could I request a review from you? Is this "the right way"?

[fare]

LGTM. Yup, that's the correct way to do things with ASDF3.