Merge pull request #6522 from peppy/fix-solo-pollution

Fix `[Solo]` attributes getting committed by blocking from NUnit runs

osu.Framework/Testing/TestScene.cs

@@ -549,6 +549,10 @@ public override void BeforeTest(ITest test)
                 if (test.Fixture is not TestScene testScene)
                     return;
 
+                bool hasSoloAttribute = test.Method?.MethodInfo.CustomAttributes.Any(a => a.AttributeType == typeof(SoloAttribute)) == true;
+                if (DebugUtils.IsNUnitRunning && hasSoloAttribute)
+                    throw new InvalidOperationException($"{nameof(SoloAttribute)} should not be specified on tests running under NUnit.");
+
                 // Since the host is created in OneTimeSetUp, all game threads will have the fixture's execution context
                 // This is undesirable since each test is run using those same threads, so we must make sure the execution context
                 // for the game threads refers to the current _test_ execution context for each test