FilterHelperTest.py: Add `FilterHelperTest` class
FilterHelperTest.py: Add `FilterHelperTest` class

This adds an additional `FilterHelperTest` class
and tests `InvalidFilterException` by raising the
exception using `FilterHelper` and tests all the
internal features of `coalib.parsing.FilterHelpers`
on a lower level.

Closes https://github.com/coala/coala/issues/4864

jayvdb/coala,jayvdb/coala,coala/coala,coala/coala,jayvdb/coala,SanketDG/coala,SanketDG/coala,coala-analyzer/coala,coala-analyzer/coala,SanketDG/coala,coala-analyzer/coala,coala/coala