Convert dotnet-format integration tests from pipeline YAML to XUnit/Helix

[MichaelSimons]

Summary

Converts the dotnet-format integration test pipeline legs (YAML + PowerShell) into MSTest integration tests that run on Helix. This eliminates redundant SDK builds and reduces PR validation cost.

Time Savings

Measured from build 1412916:

Leg	Old Leg Time	PR	CI
sdk	19m 03s	✅ Runs (saves ~8m build)	✅ Runs (saves ~8m build)
Roslyn	28m 50s	⏭️ Skipped (saves ~29m)	✅ Runs (saves ~8m build)
msbuild	17m 02s	⏭️ Skipped (saves ~17m)	✅ Runs (saves ~8m build)
project-system	12m 31s	⏭️ Skipped (saves ~13m)	✅ Runs (saves ~9m build)
efcore	15m 34s	❌ Dropped (saves ~16m)	❌ Dropped (saves ~16m)
aspnetcore	25m 34s	❌ Dropped (saves ~26m)	❌ Dropped (saves ~26m)
Total savings		~101m	~75m

AspNetCore and EfCore dropped: These repos are restore-intensive and not feasible to run on Helix. The ROI is questionable for a format validation check.

What changed

• New test project: test/dotnet-format.IntegrationTests/ using MSTest.Sdk with one test class per repo (SDK, Roslyn, MSBuild, ProjectSystem)
• Base class pattern: FormatIntegrationTestBase handles clone, restore, and format execution. Adding or removing a repo requires only adding or deleting a ~15-line class.
• Dynamic Helix sharding: PartitionByClass=true metadata causes AssemblyScheduler to discover test classes at scheduling time and create one Helix work item per class automatically.
• Priority-based test filtering: DOTNET_SDK_TEST_MAX_PRIORITY env var (set in the pipeline YAML for PR builds) and a numeric Priority property control which tests run in each context. P0 tests run in all builds (including PR validation); P1 tests (the default) run only in CI.
• Removed old infrastructure: Deleted eng/dotnet-format/ (YAML template, PowerShell verifier, CMD wrapper, RSP file) and removed pipeline references from .vsts-ci.yml / .vsts-pr.yml.

Design

Each repo is a separate test class inheriting from FormatIntegrationTestBase:

[TestClass]
public class SdkFormatTests : FormatIntegrationTestBase
{
    protected override string RepoUrl => "https://github.com/dotnet/sdk";
    protected override string Sha => "e6bc966cc3d1348265b0831c6daca23267169d8f";
    protected override string TargetSolution => "sdk.slnx";
    protected override int Priority => 0; // Runs in PR validation
}

This one-class-per-repo pattern enables:

1. Helix sharding — each class becomes its own work item (parallel execution)
2. Zero maintenance — adding or removing a repo only requires adding or deleting a class file; sharding adapts automatically
3. Extensible priority filtering — any test class can declare its priority level. The CI infrastructure sets DOTNET_SDK_TEST_MAX_PRIORITY=0 for PR builds (unset in CI = run all priorities). Adding a new priority tier requires no test code changes, only a new env var value in the pipeline.

[MichaelSimons]

@mwiemer-microsoft, @phil-allen-msft - I had copilot convert the dotnet-format integration tests to helix in order to save compute as well as make the tests just list any other test. As part of this work, we only run a subset of the tests in PR (as discussed with Phil earlier). This is WIP but wanted to share to get your early feedback.

[MichaelSimons]

/ba-g all test failures appear related to #54608