Clean up mock corpus module

[lukavdplas]

The frontend has a utility file with mock corpus and corpus field objects for unit tests. This has some issues:

• All value are defined as constants. If a test requires that you adjust some property, you need to clone these objects, or the test will affect each other.
• These definitions are quite old and don't match the (more standardised) configurations we use now. For instance, the text content field does not include multifields. The first field is a keyword field with a boolean filter.
• Most tests use mockCorpus3 and mockField2, but these kind of names are not transparent.