Add tests for kwarg overriding engine/backend from urls

[fdemmer]

in reference to #464

[github-actions]

Your PR was set to target main, PRs should be target develop
The base branch of this PR has been automatically changed to develop, please check that there are no merge conflicts.

[coveralls]

coverage: 92.955% (+0.3%) from 92.612%
when pulling 921afb1 on fdemmer:464-test-custom-backends
into a1113e4 on joke2k:develop.

[sergeyklay]

Thank you!