Test that all message names that a linter defines are added

[cfournie]

To prevent #43 from reoccurring, we should add a test to ensure that the set of message names that a linter uses in add_message calls is equivalent to the message names in the linter's msgs variable.

We could use integration tests to accomplish this, e.g. Shopify/pylint@1f85e3c

[cfournie]

Specifically, we want to be able to assemble a directory like this with functional tests for each of our custom rules and adapt this functional test generation code to generate py.test tests for each set of functional test files.