Add support for External Store for unit tests
This allow running PHP tests with External Store DB configured.
This means it needs to do the unittest_ prefixing similar to the main
It could do resetting (truncating the tables) similar to the tablesUsed
functionality. However, if External Store is truncated unconditionally,
revision, etc. also needs to be. This can cause performance issues.
I was thinking of making resetting ES optional, but it wasn't needed
to pass any of the core tests, so I just left it out entirely for now.