Use pytest to run search engine backend tests

[alastair]

Description
Instead of writing a custom script to perform tests for the solr backend, use pytest. We specify a new mark which allows us to select only these tests, by default they are not included with unit tests.

Todo:

• check that they give the same results as the normal script
• now that we run in a test harness, the database is empty. Populate it with some test items
• restore the debug code which writes text files with results