Assert usage cleanup

Following bandit warning about usage of assert in code, this issue is used to track down the cleanup of these statements found in beat.core.

Depending on how they are used, the following actions can be done:

1. Turn the statement into an exception
2. Handle the case gracefully
3. Mark as not to get worried about

The current state of the code base is usually 3 which should be cleaned up.

All tests in this repository have been cleaned to use the testing framework corresponding tools