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

[imonteroperez]

Highlights

• Follow up from Trivial support for ipv6 docker-fixtures#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.
• CI errors obtained on the https://ci.jenkins.io/job/Core/job/acceptance-test-harness/job/PR-1510/ seems to be the same than in master branch

Submitter checklist

Beta Give feedback

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

[imonteroperez]

@jtnord all test failures are unrelated with the proposed changes, asking for a merge :-)

[jtnord]

failures all seem to be related to #1526

[uhafner]

This breaks the API for downstream plugins!

See jenkinsci/warnings-ng-plugin#1774 and compile error:
See https://github.com/jenkinsci/warnings-ng-plugin/actions/runs/9809905482/job/27088840082?pr=1774