Adapt some tests to make it feasible to run it under IPv6 environments

[imonteroperez]

Highlights

• Follow up from https://github.com/jenkinsci/docker-fixtures/pull/73
• Updates some tests to use the mentioned IPv6 docker-fixtures support as well as some required additional refactoring (i.e. URL# methods IPv6 compliant, etc.)

Testing done

• Tested under dedicated IPv6 environment and no issues.

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
- [x] Link to relevant pull requests, esp. upstream and downstream changes
- [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue