[wasm][coreclr] Disable structinreg P/Invoke test on platforms without native test assets

[radekdoulik]

Note

This PR description was AI/Copilot-generated.

Summary

Disables the structinreg P/Invoke test until native linking is ready on wasm/coreclr. The test uses [DllImport("jitstructtests_lib")] which requires native test assets.

Tracked by #123946

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[Copilot]

Pull request overview

Disables the structinreg System V bring-up test entrypoint on platforms that currently don’t support native test assets (required due to DllImport("jitstructtests_lib") usage in the test suite), aligning with the infra limitations tracked in #123946.

Changes:

• Adds an [ActiveIssue] guard to skip the test when PlatformDetection.PlatformDoesNotSupportNativeTestAssets is true.
• Adds a TestLibrary using to support the new platform detection reference.

[Copilot]

using TestLibrary; is currently unused because the attribute references TestLibrary.PlatformDetection with a fully-qualified name. This will introduce an unnecessary using directive (and can trigger CS8019 in some builds). Remove the using, or simplify the attribute to use typeof(PlatformDetection) / nameof(PlatformDetection.PlatformDoesNotSupportNativeTestAssets) so the using is actually needed.