Fix broken company-jedi-location function.

Normally find object definition at point.
Successful obtain module_path, line pair at point.
Extend company-jedi-point with additional params.

Two new params are current company-prefix and company-arg. Server side
api not implemented yet. Server side ert tests will fail with internal
server error.

Ert tests refactoring. Tests can create buffer context with helper
function.