Migrate microsoft/vstest onto the flat flow

[premun]

• Run the migration tool in dry run (to backup the subscription changes)

  dotnet run -- migrate --output migration-log.json [repo]
  

• Share the contents of the migration log in this issue
• Verify the migration log - basic sanity check
• Run the migration tool in full mode

  dotnet run -- migrate --perform-updates [repo]
  

• Go to https://maestro.dot.net/subscriptions and perform a quick sanity check on the subscriptions of this repo
• Trigger the backflow subscription
• Validate the opened backflow PR
  • Updates should include mostly be version files (like we did an old dependency flow), eng/common files
  • Updates should not include VMR package versions. This repo should not receive updates from VMR builds. If so, hold on backflow merge.

[premun]

Migration log:

info: Migrating branch main of https://github.com/microsoft/vstest to flat flow...
info: Found 0 outgoing and 4 incoming subscriptions for vstest
info: Processing incoming subscription 58a671c7-126e-45c3-8666-08d91b0a8694 https://github.com/dotnet/arcade -> https://github.com/microsoft/vstest...
info: Disabled subscription 58a671c7-126e-45c3-8666-08d91b0a8694 https://github.com/dotnet/arcade -> https://github.com/microsoft/vstest
info: Processing incoming subscription af712b65-5e6e-4ab3-9c28-08db4c03ab45 https://github.com/dotnet/source-build-reference-packages -> https://github.com/microsoft/vstest...
info: Disabled subscription af712b65-5e6e-4ab3-9c28-08db4c03ab45 https://github.com/dotnet/source-build-reference-packages -> https://github.com/microsoft/vstest
info: Processing incoming subscription 86950fd0-829d-4c2e-7e88-08db4ca58899 https://github.com/dotnet/source-build-externals -> https://github.com/microsoft/vstest...
info: Disabled subscription 86950fd0-829d-4c2e-7e88-08db4ca58899 https://github.com/dotnet/source-build-externals -> https://github.com/microsoft/vstest
info: Processing incoming subscription 88c2493e-8531-4427-9777-08dc0c5533ad https://github.com/dotnet/diagnostics -> https://github.com/microsoft/vstest...
info: Disabled subscription 88c2493e-8531-4427-9777-08dc0c5533ad https://github.com/dotnet/diagnostics -> https://github.com/microsoft/vstest
info: Arcade subscription is for .NET 9 Eng channel. Excluding 3 Arcade assets in backflow subscription
info: Creating a backflow subscription for https://github.com/microsoft/vstest
info: Created a backflow subscription 6240778c-085c-4e63-ba23-e3ede3cbb6c8
info: Creating a forward flow subscription for https://github.com/microsoft/vstest
info: Created a forward flow subscription 3e0254e3-40df-494a-8987-de1ada03df2a
info: Repository vstest successfully migrated

[premun]

Backflow PR opened but needs exclusions

[premun]

New backflow PR: microsoft/vstest#15061