Cover strictly 100% of the code

[ichiriac]

Improve the coverage with mocha :

• 100% of lines of code
• 100% of statements
• > 95% of branches

[cseufert]

There are a lot of branches that are not straightforward to test, as the branch will not be exercised unless the mode is changed from error to warning (for the expect() functions, and random AST nodes are removed to trigger all these cases (lots of expected brackets, arrows, etc). I wonder if writing a test driver that drops each symbol in each test, and takes a snapshot of the output would work here?