Re-enable format test

[mthalman]

The test for dotnet-format is currently disabled: https://github.com/dotnet/sdk/blob/c55992302499b7b9d8a1cf22a4a55bd47bb797cc/src/SourceBuild/content/test/Microsoft.DotNet.SourceBuild.Tests/DotNetFormatTests.cs#L23-L25

This should be re-enabled once dotnet/roslyn#76797 is fixed.