Testing: add testing data and integration tests

[jsheunis]

There are existing unit tests, although altogether these are probably not sufficient as a full test suite. A few integration tests are needed, especially to test complicated and/or recently developed cases e.g. the addition of top_display and additional_display metadata to a catalog. This extension uses pytest but tests were developed without considering the migration of datalad's tests from nose to pytest. Perhaps there are now reusable snippets of code from datalad.

While working on this issue, take note of the relevant information in these issues:

• Delineate and structure package data #100
• Update template dataset with metadata for demo + local development #50

To do:

• create test data that includes more complicated and possibly whole-catalog metadata, in order to allow testing more complicated catalog add operations as well as a full catalog generation integration test.
• write tests for more recent/complicated metadata add operations
• write a full catalog generation integration test
• check test modules to see if redundancy or inefficiencies can be removed and if decorators/snippets/utilities/etc can be imported from datalad's updated testing framework.

[jsheunis]

Closed by #309