Create a test suite for the Mix tasks

[alco]

No description provided.

[bbense]

I've made a first pass at a simple test.

https://github.com/bbense/benchfella/tree/mix_task_test

[alco]

Hey @bbense,

Sorry for not getting back to your earlier. The issue with multiple invocations of Benchfella needs to be solved at the architectural level. Currently it is invoked from an at_exit callback and the code doesn't have any special measures guarding against repeated initialization of ETS tables and other things.

As for testing the tasks, the primary focus should be on verifying that all options work. The output should only be checked for basic clues that indicate that the expected tests are running. Since there will be more than one output format and the current format is likely to change in the future, I wouldn't advise comparing the whole output against a string.