This commit adds a broken test, which is meant to expose a flaw in the
constructor of `Cask::Config`.
That (broken) test still passes because there’s also a flaw in our
test helper code.
The helper flaw happens to neutralize the `Cask::Config` flaw.
Previously, the JSON-based cask config loader was untested.
This commit changes the interface to accept a string, making the loader
easier to test. The commit also adds a test.