Migrate the test project to the new dotnet test experience before xunit v4 #111

xunit.v3 4.0.0 cannot be merged as a version bump. It needs a migration first, and this issue is the hold's tracking issue (referenced from .github/renovate.json5).

What fails

Renovate proposed the bump in #108. Both .NET test jobs failed, identically on CI and on a local Mac:

/home/runner/.nuget/packages/microsoft.testing.platform.msbuild/2.3.3/buildMultiTargeting/Microsoft.Testing.Platform.MSBuild.targets(320,5):
error : Testing with VSTest target is no longer supported by Microsoft.Testing.Platform
on .NET 10 SDK and later. If you use dotnet test, you should opt-in to the new
dotnet test experience.

See https://aka.ms/dotnet-test-mtp-error.

Why

xunit.v3 4.0.0 depends on xunit.v3.mtp-v2 [4.0.0, 4.0.0], which pulls Microsoft.Testing.Platform 2.x. MTP 2 drops the VSTest bridge on the .NET 10 SDK, so the current dotnet test invocation has nowhere to run.

This is not the same failure mode as the Verify family coupling documented in test/FactorioTools.Test/FactorioTools.Test.csproj. That one builds cleanly and dies at runtime. This one fails at build time, loudly, which is the easier kind.

What the migration involves

At minimum:

• Opt the test project into the new dotnet test experience (a dotnet.config with a [dotnet.test.runner] section, or the equivalent MSBuild property).
• Replace the VSTest-only arguments in .github/workflows/ci.yml. Both .NET jobs pass --logger "console;verbosity=normal", which is VSTest syntax that MTP does not accept. Check --no-build too.
• Re-check the "Check no Verify snapshots drifted" step still sees the same working-tree changes, since it depends on the test run actually happening.
• Confirm coverlet.collector 10.0.1 still collects under MTP, or swap it for the MTP coverage path.
• Re-run both matrix jobs (default settings and UseLuaSettings=true).

What is held meanwhile

.github/renovate.json5 holds xunit.v3 and xunit.runner.visualstudio below 4.0.0, so patches within 3.x keep flowing and the bot stops re-proposing the major every week. Raise the ceiling as part of this migration, not before.

Verify.XunitV3 32.0.0 was merged separately in #106 and is unaffected - both .NET test jobs passed on it.