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

[MichaelSimons]

Summary

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

Time Savings

Measured from build 1412916 with timing instrumentation:

Leg	Total Leg Time	SDK Build Time (eliminated)
sdk	19m 03s	8m 02s
Roslyn	28m 50s	8m 26s
project-system	12m 31s	8m 43s
msbuild	17m 02s	8m 25s
efcore	15m 34s	7m 19s
aspnetcore	25m 34s	8m 27s
Total	~118 min	~49 min

In PR validation: ~107 minutes of machine time saved.

• Previously all 6 legs ran in PR (~118 min total machine time, ~29 min critical path)
• Now only the SDK leg runs in PR, and without the SDK build (~19m - 8m = ~11m)
• Savings: ~99 min from dropping 5 legs + ~8 min from eliminating the SDK build in the remaining leg

In CI (non-PR): ~49 minutes of SDK build time eliminated.

• All 6 legs still run but use the pre-built SDK already deployed to Helix instead of rebuilding it each time

What changed

• New test project: test/dotnet-format.IntegrationTests/ with one test class per repo (SDK, Roslyn, MSBuild, ProjectSystem, AspNetCore, EfCore)
• Base class pattern: FormatIntegrationTestBase handles clone, restore, and format execution. Adding or removing a repo requires only adding or deleting a ~20-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.
• PR vs CI filtering: DOTNET_SDK_PR_VALIDATION env var and RunInPRValidation property control which tests run in PR validation. This infrastructure is extensible — any future test class can opt into or out of PR runs by setting this property, enabling broader use of PR-conditional test filtering beyond format tests.
• 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:

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 bool RunInPRValidation => true;
}

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 PR filtering — the RunInPRValidation pattern can be adopted by other test classes in the future to conditionally exclude expensive tests from PR validation

[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.