Arcade integration for MTP runs test projects one at a time for multi tfm

[nohwnd]

Describe the bug

2026-03-09T15:32:41.1697193Z Tests succeeded: D:\a_work\1\s\artifacts\bin\Microsoft.TestPlatform.Acceptance.IntegrationTests\Release\net9.0\Microsoft.TestPlatform.Acceptance.IntegrationTests.dll [net9.0|x64]
2026-03-09T15:32:41.1721154Z Running tests: D:\a_work\1\s\artifacts\bin\Microsoft.TestPlatform.Acceptance.IntegrationTests\Release\net48\Microsoft.TestPlatform.Acceptance.IntegrationTests.exe

Version used

IDK, close to latest arcade on the latest channel.

Steps To Reproduce

Have project with 2 tfms, run it via arcade

https://dev.azure.com/dnceng-public/public/_build/results?buildId=1326478&view=logs&j=7c8326b9-0a5f-532a-e6de-db8515c72d9a&t=a2e3061e-1c43-5369-d1c5-db1b5bad61d7

[nohwnd]

2026-03-09T12:20:44.4881094Z Tests succeeded: D:\a_work\1\s\artifacts\bin\Microsoft.Testing.Platform.MSBuild.UnitTests\Release\net8.0\Microsoft.Testing.Platform.MSBuild.UnitTests.dll [net8.0|x64]
2026-03-09T12:20:44.4912599Z Running tests: D:\a_work\1\s\artifacts\bin\Microsoft.Testing.Platform.MSBuild.UnitTests\Release\net9.0\Microsoft.Testing.Platform.MSBuild.UnitTests.dll [net9.0|x64]

same for testfx see build https://dnceng.visualstudio.com/7ea9116e-9fac-403d-b258-b31fcf1bb293/_apis/build/builds/2921903/logs/157

[nohwnd]

S:\p\vstest [main ≡]> ./test.cmd -integration -projects S:\p\vstest\test\Microsoft.TestPlatform.Acceptance.IntegrationTests\Microsoft.TestPlatform.Acceptance.IntegrationTests.csproj

Running tests: S:\p\vstest\artifacts\bin\Microsoft.TestPlatform.Acceptance.IntegrationTests\Debug\net9.0\Microsoft.TestPlatform.Acceptance.IntegrationTests.dll [net9.0|x64]

This should show both tfms running, I have more than 1 core in my computer. It shows just 1