Remove MongoDB testing dependency

[josebolos]

The current testing suite for pa11y-webservice requires mongodb to be available and properly set up for the tests to run.

This is usually not much of a problem when running on a CI environment as we've already done the hard work to configure all that, but it makes it much harder to make changes to the project and test things locally. It rises the barrier for contributing to the project.

There's also the risk that, if you use a production DB to run the tests (in order to avoid setting up one local mongoDB instance) all the data will be wiped in the process. This is hostile to the user and fairly unforgiving.

I think that we should rework the tests so they don't depend on a MongoDB instance to be up.