binary-search: clean up

[arcuru]

Cleans up a few issues remaining from adding the binary-search exercise.

• Removed extra .cache files
• Altered filenaming system to match other exercises
• Updated tests to match common json

[arcuru]

I'll just note that without this patch, anyone doing the exercise needs to name their solution example.py unless they modify the test file.

[Dog]

Thanks 👍

[kytrinyx]

We're missing binary-search from the "exercises" key in the config.json.