tests: Disable tests that would rebuild the world due to grafting.

The Perl replacement added in d8173f21 leads some tests to involve
grafting, which in turn triggers derivation builds that rebuild the
world.  Work around that.

* tests/packages.scm: Add call to '%graft?'.
("package-derivation, direct graft")
("package-cross-derivation, direct graft"): Comment out.