feat: remove different testing modes

I've seen a few users getting confused as to why echidna is telling them that there are no tests to be found. It's usually the case that they did not include `--test-mode assertion`.

Suggestion:
Remove different testing modes. Assertion mode should be the default. The normal mode (where functions with signature "echidna*() returns (bool)" are being used can just be included, i.e. if Echidna finds functions that meet that requirement, use the normal mode on those. Optimization mode could be done automatically for functions that return a comparable value (uint, int), i.e. "echidna*() returns (uint)". This might remove some of the friction for onboarding new users.