Add a regression test for incremental builds

PR #66 fixes incremental builds by adding WriteOnlyWhenDifferent="true" to the WriteLinesToFile call in CreateDfeAnalyticsDesignTimeServicesClass. It shipped without a regression test.

Both test projects are plain C# xUnit and there is no MSBuild/targets test harness to extend. Adding one means building fixture projects from inside a test run and diffing timestamps, which is genuinely new infrastructure — not worth holding a one-attribute fix behind.

If we want the guard, the cheap shape is:

• a fixture .csproj under tests/ that Imports the targets file directly from Package/buildTransitive/ rather than via a packed nupkg
• build it twice with dotnet build
• assert LastWriteTimeUtc on the generated .g.cs is unchanged after the second build

Importing the targets directly is what keeps it runnable in CI without a pack-and-restore cycle.

The failure mode is slow builds rather than incorrect output, so a re-regression would be cheap and loud — this is a nice-to-have, not urgent.

Start in the existing test projects and inspect the targets under Package/buildTransitive/. Add a fixture .csproj under tests/ that imports the targets directly, build it twice with dotnet build, and compare the generated .g.cs file's LastWriteTimeUtc. Done means the timestamp is unchanged after the second build and the test runs in CI without a pack-and-restore cycle.