Create test for singleton factory, to ensure it properly instantiates…

… the singletons

TJC.Singleton.Tests/Mocks/Valid/MockSingletonInstantiated.cs

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+namespace TJC.Singleton.Tests.Mocks.Valid;
+
+internal class MockSingletonInstantiated : SingletonBaseClass<MockSingletonInstantiated>, IIdentifier
+{
+    private MockSingletonInstantiated() {}//=> IsInstantiated = true;
+
+    public Guid Id { get; } = Guid.NewGuid();
+
+    //public static bool IsInstantiated { get; private set; }
+}

TJC.Singleton.Tests/Tests/Instantiated/IsInstantiatedTest.cs

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+using TJC.Singleton.Factories;
+
+namespace TJC.Singleton.Tests.Tests.Instantiated;
+
+[TestClass]
+public class IsInstantiatedTest
+{
+    [TestMethod]
+    public void SingletonGetInstantiatedAfterBeingReferencedTest()
+    {
+        // MockSingletonInstantiated can only be used in this test and nowhere else, since object instances can persist between tests
+        Assert.IsFalse(MockSingletonInstantiated.IsInstantiated, $"{nameof(MockSingletonInstantiated)} was already instantiated");
+        SingletonFactory.InstantiatedAll(trace: true);
+        Assert.IsTrue(MockSingletonInstantiated.IsInstantiated, $"{nameof(MockSingletonInstantiated)} is not instantiated");
+    }
+}