Don’t use Sourcery log handler mock where concurrent access is possible

[lawrence-forooghian]

We seem to have had an increased number of crashes in the test suite recently, e.g. this one. I briefly investigated these and I think it’s due to concurrent access of mutable shared state in InternalLogHandlerMock.

So, don’t use the Sourcery mock in these situations, and document why.

I suspect that I introduced these crashes in 0e8515c, which adds logging to the Callback struct. Its messages are logged from multiple threads.