Tests: add Windows into the RNG path

[compnerd]

This adds Windows to the SystemNumberGenerator condition as well, which
is needed to get the tests to build on Windows.

[One line description of your change]

Checklist

• I've run tests to see all new and existing tests pass
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

If you've made changes to gyb files

• I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request

Motivation:

[Explain here the context, and why you're making that change. What is the problem you're trying to solve.]

Modifications:

[Describe the modifications you've done.]

Result:

[After your change, what will change.]

[Lukasa]

I can confirm I’ve audited the Swift code, and that the Windows implementation of SystemRandomNumberGenerator is suitable for use as a CSPRNG.