Fixes test container paths so they work on Windows

[mhairifin]

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This PR patches Wiremock to force container paths into posix formatted paths when files are added to the container, so that the test containers work on Windows.

References

• https://github.com/wiremock/python-wiremock/issues/103

Submitter checklist

• [ ] Recommended: Join WireMock Slack to get any help in #help-contributing or a project-specific channel like #wiremock-java
• [x] The PR request is well described and justified, including the body and the references
• [x] The PR title represents the desired changelog entry
• [x] The repository's code style is followed (see the contributing guide)
• [x] Test coverage that demonstrates that the change works as expected
• [ ] For new features, there's necessary documentation in this pull request or in a subsequent PR to wiremock.org