Complete automated tests for Filter#toString

[chrmod]

Lets be sure that NetworkFilter#toString is able to correctly export all supported filter types.

[chrmod]

when working on https://github.com/ghostery/adblocker/pull/3681 I've noticed some unexpected behaviour - I think simple filter like foo.com^ was converted to something quite longer.

[chrmod]

require('@cliqz/adblocker').NetworkFilter.parse('foo.com^').toString()