Use busted lua framework for functional tests

[evgenykor]

We may better improve tests if we use busted framework for unit testing. This would basically mean rewrite all tests with it but as a result it will be much cleaner IMHO. What people think about it?

[somesocks]

@evgenykor I don't know anything about testing frameworks for Lua, is busted the preferred standard for testing today?

I think its fine to switch frameworks if its a good one, but I'd suggest making a copy of the test cases to migrate, and running both old test cases and the new busted ones for a while, until we have full coverage.