[WIP] [popup] add specs for calcPossibleDrawingParams

Fixes #1006
@narqo I'm agree with testing behaviour instead of code. But there is hard to see what should be tested: https://github.com/bem/bem-components/blob/v2/common.blocks%2Fpopup%2F_target%2Fpopup_target.js#L107

// cc @dfilatov