Add ActiveIssue to the new self_override_generic test

[MichalStrehovsky]

Also update ActiveIssue on the self_override5.il test which is yet another thing the batch update in #123248 did wrong.

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[Copilot]

Pull request overview

Updates IL-based loader/classloader tests to use the correct inline ActiveIssue metadata for NativeAOT, aligning with the post-#123248 approach of expressing test exclusions directly in the test source.

Changes:

• Fixes self_override5.il to use the issue URL (instead of descriptive text) in the ActiveIssueAttribute payload.
• Adds Microsoft.DotNet.XUnitExtensions / TestLibrary extern references and an ActiveIssueAttribute to the new self_override_generic.il test to gate it on NativeAOT.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/tests/Loader/classloader/MethodImpl/Desktop/self_override5.il	Corrects ActiveIssueAttribute argument to the canonical GitHub issue URL for NativeAOT gating.
src/tests/Loader/classloader/MethodImpl/Desktop/self_override_generic.il	Adds required extern assemblies and introduces ActiveIssueAttribute for NativeAOT gating on the generic MethodImpl override test.

[MichalStrehovsky]

/azp run runtime-nativeaot-outerloop

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[MichalStrehovsky]

/ba-g the macos globalization issue was fixed and the underlying issue is now closed, so it doesn't match.