Add mock and integration tests for client RPCs#31

Marenz wants to merge 2 commits intofrequenz-floss:v0.x.xfrequenz-floss/frequenz-client-marketmetering-python:v0.x.xfrom

Marenz:add/client-testsMarenz/frequenz-client-marketmetering-python-1:add/client-testsCopy head branch name to clipboard