tests: skip the tests a faked artifacts package cannot run

The gated medium chain builds its package with FAKE_ARTIFACTS=true, so the
check-mk-agent packages in it are placeholder files. The three tests marked
here bake an agent, or read byte-compiled config out of the package, and
fail on that placeholder rather than on anything real.

skip_if_faked_artifacts already exists and is enforced through
--package-contains-faked-artifacts, so this only applies it where it
belongs. Nothing changes for the heavy chain, which builds real packages.

CMK-32643

Change-Id: I89bdae67f125fce528f9f8e79c6862b1ff78d32b