WinFormsTestApp: purge stale lock/obj to drop Microsoft.WindowsDesktop.App.Ref 3.1.0 (fix CG Critical from #5482)

[gladjohn]

Fixes- CG alert

Changes proposed in this request
Component Governance flags a Critical alert for Microsoft.WindowsDesktop.App.Ref 3.1.0 during the WinForms test app build. There is no explicit reference in the repo; the alert results from restore artifacts/lockfile behavior. This PR prevents creation of a per-project lockfile for the test app so new builds resolve against current framework refs and do not surface the legacy package.

Component detection - https://identitydivision.visualstudio.com/IDDP/_build/results?buildId=1535830&view=logs&j=5af50cc2-dfd2-559f-4713-be7402902cc5&t=154cf91c-b1f3-57ab-1919-56f7d79b7fe3&l=2456

Rationale

• The test app targets net8.0-windows* and uses framework refs (WinForms/WPF).
• CG was reading an older dependency resolution from lockfile/restore artifacts and attributing it to this project.
• Disabling the per-project lockfile for this test app ensures future restores don’t reintroduce the legacy Microsoft.WindowsDesktop.App.Ref 3.1.0 into the scanned graph.

Testing
devapp

Performance impact
none

Documentation
n/a