New Windows installer add unit tests

[julien-lebot]

What does this PR do?

This PR adds unit tests for the next-gen Windows installer. It uses xUnit with Moq for the mocking framework, and AutoFixtures for generating sample data.

Motivation

Better test coverage.

Additional Notes

This PR also introduces an interface ISession that proxies the real Session object for unit testing purposes.

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes

Reviewer's Checklist

• [ ] If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
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• [ ] A release note has been added or the changelog/no-changelog label has been applied.
• [ ] Changed code has automated tests for its functionality.
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