Add checks to logger unit tests

[martygrant]

There is a range of unit tests in test/unit/logger/logger.cpp to test logging functionality, however none of the tests actually check the output is actually received at the given destination. These should be modified to properly test this works correctly.

This was highlighted by #1748 (comment)

[martygrant]

I didn't see that the FileSink tests are running an assert in their tear down function, so this issue is no longer needed.