ExtractPlist: expand tests

This extracts the logic for creating a duplicate cask with a different
artifact URL into a separate method, so we can properly test it. It's
possible to exercise this in `ExtractPlist.find_versions` tests to
some degree but isolating this code allows us to check the returned
cask and ensure that this works like it should.

This also expands `ExtractPlist` tests, bringing coverage to 100% for
both lines and branches. I removed an existing `find_versions` test
case which seemed to only check that `Cask::CaskLoader.load` works, as
it wasn't testing `ExtractPlist`. Similarly, I removed the
`UnversionedCaskChecker` expectations, as these tests should arguably
be checking the output from `ExtractPlist` methods, not what's
happening inside.