tests: addition of test_example_app.py

The package contains `examples/app.py` example application, but there is no test
for it in `tests/test_example_app.py`.  It should be added.  See existing examples:
- https://github.com/inveniosoftware/invenio-records/blob/master/tests/test_example_app.py
- https://github.com/inveniosoftware/invenio-marc21/blob/master/tests/test_example_app.py
