tests: addition of test_example_app.py

[tiborsimko]

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