Reuse lexer and parser between tests

[ris58h]

Both Lexer and Parser have setInputStream method so they can be reused. We could add an option to reuse them between tests of the same grammar to make sure it works properly.

[KvanTTT]

I'm not sure if it makes big sense because I suspect it almost doesn't affect performance.

[ris58h]

It's not about performance but verification. I had an issue with JavaScript lexer that affected my project. Reusing lexer and parser will allow us to test such case.