EZP-21670: Tests additional refactoring

JIRA issue: https://jira.ez.no/browse/EZP-21670

Implementing additional refactorings. 
Better be reviewed after merging https://github.com/ezsystems/ez-js-rest-client/pull/25
Built on this PR's base, because we are using custom matchers infrastructure introduced there.

 - [x] custom matcher for testing Connection in ConnectionManager
 - [x] switching from `expect().toBe(...)` constuct for each of the arguments to `expect().toHaveBeenCalledWith(...)` in almost every call in every service