Condition out the native tests in VMR

[NikolaMilosavljevic]

Fixes: #13188

System.Windows.Forms interop tests depend on NativeTests project, which doesn't build in VMR.

This PR just conditions the test project to not build in VMR. Additional work would be needed to enable these tests in VMR. The remaining work is likely best suited for someone with domain knowledge.

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.35008%. Comparing base (e48889f) to head (1ce0ac6).
Report is 21 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #13187         +/-   ##
===================================================
+ Coverage   61.34882%   61.35008%   +0.00125%     
===================================================
  Files           1547        1547                 
  Lines         158479      158479                 
  Branches       14751       14751                 
===================================================
+ Hits           97225       97227          +2     
+ Misses         60553       60550          -3     
- Partials         701         702          +1     

Flag	Coverage Δ
Debug	61.35008% <ø> (+0.00125%)	⬆️
integration	10.73185% <ø> (-0.00104%)	⬇️
production	39.21770% <ø> (+0.00207%)	⬆️
test	95.67947% <ø> (ø)
unit	36.65017% <ø> (+0.00416%)	⬆️

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[NikolaMilosavljevic]

@Tanya-Solyanik who can take a look at this from WinForms team? I'd also need help merging as I don't have sufficient permissions in this repo.