Rewrite high-level color override tests in core

The dynamic JSON, XML, and YAML services have service tests for color override functionality which is in BaseService. The low-level functionality is checked in `coalesceBadge` which is fine, though there probably should be a higher-level test for BaseService.

These tests should be removed from `dynamic-*.tester.js`.

Ref: https://github.com/badges/shields/pull/3066#discussion_r258731711

Start with the color override tests in dynamic-*.tester.js and compare them with the low-level coalesceBadge coverage. Read BaseService to determine the appropriate higher-level test location, then remove the duplicated service tests and ensure the remaining coverage still verifies the intended color override behavior.

Written by the indexing model from the issue text.