Fix ep-engine test issues when run under ThreadSanitizer

A number of ep-engine 3.x unit tests have issues when run under ThreadSanitizer - i.e. the test(s) themselves are buggy or have racy behaviour.

We should fix these issues, firstly so we know the tests are operating correctly and secondly to we can "see the wood for the trees" and identify actual issues with the product code.

To be clear: the scope of this MB is to fix issues only with our tests and test harnesses. Actual issues with production code will not be addressed here.