This is a good test but it only tests one of the three ways in which we can match a file given the regexp in `Cask_file_source?`

```ruby
Cask.load("./relative/path/to/some-cask.rb") # covered by test
Cask.load("/absolute/path/to/some-cask.rb") # needs test
Cask.load("file:///url/style/path/to/some-cask.rb") # needs test
```

In fact because file:// style urls can go through the URL code path we may want to consider cutting that out as being considered a "file source" and just go with the first two. What do you think?