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

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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