refactor(config): junit/coverage plugin configs

This stuff belongs to these plugins:
https://github.com/karma-runner/karma-coverage/commit/2ac2f71a1af13ff40674b4884173b6b487bf4f19
https://github.com/karma-runner/karma-junit-reporter/commit/ff21972c735f160e66085f8fedaa81c8efbb0e0f
refactor(config): remove the BC warnings
refactor(config): extract normalizeUrlRoot
fix(config): use polling by default

This should have been set to `true` in 18514d63534c82094b231eb1e0b0e41011519183. It's reversed.