Add tests for `:scope` and `:root` selectors

[run1t]

As described in https://github.com/estools/esquery/pull/61 I added the tests for root an scope. Tell me If I need to add more tests.

I also addded a specific test for the use case described in this issue: https://github.com/phenomnomnominal/tsquery/issues/23

[run1t]

This issue has been open for 8 days now. Does someone can take a look at it please ? 😃

[EduardoRFS]

wow thx, i really need that

[run1t]

any news ?