Refactor tests that mutate the global log object

[lucianjon]

There are some tests that mutate the global log object, using this helper function https://github.com/movio/bramble/blob/main/instrumentation_test.go#L14. Specifically it gives a writer to the log object that is closed at the end of the test. This can cause other tests that run code which logs to print out errors like this: Failed to write to log, io: read/write on closed pipe.

[pkqk]

The testing library now has a Cleanup hook which could be used to restore the original output.