TreeIterator: Tests and Fixes

**Work In Progress.**

Continuing to improve our tests. This time — `TreeIterator`.

Implementing tests according to `Mozilla AST`. Found one unexpected behaviour.
We use this construction a lot, primary in test, but if we would add a new option or remove one in the future, we would have to redo it everywhere.

Maybe we should introduce static method in `StringChecker` class which would serve as decorator to `esprima.parse`.