Add Copilot skills for XAMLTest and MTP test runs

[Keboo]

Why

This branch still carries useful repo-specific testing guidance even though the numeric up-down product change was reverted. The goal of this PR is to keep the Copilot skills that encode how this repository wants UI tests and MTP/TUnit test runs handled.

What changed

• add a xamltest-ui-tests skill for writing runtime WPF coverage in MaterialDesignThemes.UITests
• add an mtp-test-running skill that documents the repo's Microsoft Testing Platform + TUnit command shape
• capture the workflow rule to avoid UI tests unless they are necessary and, when they are needed, run only the affected tests
• include concrete examples that point at existing test classes so the guidance stays usable after the revert

Notes

The important repo-specific detail is that these test projects use TestingPlatformDotnetTestSupport, so runner arguments must be passed after the extra -- when invoking dotnet test.