Try to force .NET 9 for testing via global.json

Author: rnelson

.github/workflows/dotnet.yml

@@ -28,6 +28,3 @@ jobs:
       run: dotnet build --no-restore
     - name: Test
       run: dotnet test --no-build --verbosity normal
-      with:
-        dotnet-version: |
-          9.0.x

test/SlowDown.Tests/global.json

@@ -0,0 +1,6 @@
+{
+	"sdk": {
+		"version": "9.*.*",
+		"rollForward": "latestPatch"
+	}
+}