[DDC-3414] Add test for "Joining on a table with inheritance produces badly formed ON clause"

Add a test for a bug report. The test is only simple with no assertions, since the SQL generated by Doctrine is invalid and generates an exception in SQLite.
Yeah it does seem like a bit of a pointless test with no assertion!

I was worried that checking the SQL would make the test too fragile, or that different drivers would produce different SQL - or should I skip the test for other drivers?

Or, I can make it insert data and retrieve it, asserting that everything went as expected?

I've not contributed to many open-source projects, so sorry if you need to kick me in the right direction :stuck_out_tongue: