Extract bootstrap/E2E test infrastructure into new EndToEndTestSupport shared library

[Copilot]

Context

Bootstrap-specific test infrastructure was coupled to UnitTests.Shared, which mixed general test helpers with E2E/bootstrap concerns.
This refactor introduces a dedicated shared library for bootstrap/E2E support and updates E2E callers to consume it directly.

Changes Made

• New shared library for bootstrap/E2E

  • Added src/EndToEndTestSupport/Microsoft.Build.EndToEndTestSupport.csproj.
  • Moved bootstrap wiring into this project:
    • BootStrapMsBuild.props import
    • MSBuild.Bootstrap build-order dependency
    • assembly attributes for bootstrap/artifacts paths

• Moved bootstrap-owned types out of UnitTests.Shared

  • Moved:
    • BootstrapLocationAttribute
    • ArtifactsLocationAttribute
  • Added EndToEndTestUtilities with:
    • ExecBootstrapedMSBuild(...)
    • bootstrap path/version properties previously exposed via RunnerUtilities

• Narrowed UnitTests.Shared scope

  • RunnerUtilities now retains only non-bootstrap process helpers (ExecMSBuild, RunProcessAndGetOutput, etc.).
  • Removed bootstrap-specific members from RunnerUtilities.

• Updated E2E callers + references

  • Added EndToEndTestSupport project references to 5 test projects:
    • Build.UnitTests
    • BuildCheck.UnitTests
    • MSBuild.UnitTests
    • Tasks.UnitTests
    • Build.OM.UnitTests
  • Updated call sites from:

    RunnerUtilities.ExecBootstrapedMSBuild(...)

    to:

    EndToEndTestUtilities.ExecBootstrapedMSBuild(...)

Testing

Existing E2E test projects were updated to compile and run against EndToEndTestUtilities; no new test infrastructure patterns were introduced.

Notes

This keeps bootstrap/E2E ownership centralized and leaves UnitTests.Shared focused on general-purpose test infrastructure.

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[dotnet-policy-service]

Hello @@copilot, I noticed that you’re changing an .swr file or any file under src/Package/MSBuild.VSSetup.. Please make sure to validate this change by an experimental VS insertion. This is accomplished by pushing to an exp/* branch, which requires write permissions to this repo.