Add automatic secondary index testing to CI matrix

[AndrewSisley]

We've had quite a lot of bugs recently where using an index returns different results compared to if the index did not exist at all. It should be quite easy to automatically test this.

Add some magic to the test suite so that (given an env var) indexes are added to all fields in all collections before executing requests and add a job to the CI to run it in every PR.

This will not replace the need for many secondary index tests as the proposed magic will not test that the index is actually used, but it will make sure they always return the correct results (assuming the non-index tests are good :)).