fix equality issue in DependencyContextLoader (#2379)

When checking if the provided assembly is the Entry Point Assembly, we
previously just checked if the AssemblyNames were equal, but it turns
out AssemblyName doesn't implement Equals, so it was using Reference
Equality, which fails. This change uses Assembly.Equals, which has an
Equals implementation that works.

Also adds some tests to ensure it's working.

This unblocks scenarios where the EntityFramework `dotnet-ef` command
was trying to read DependencyContext.Default but receiving a null
reference.

TestAssets/TestProjects/DependencyContextFromTool/Program.cs

@@ -0,0 +1,15 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+
+namespace DependencyContextFromTool
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            Console.WriteLine("Hi!");
+        }
+    }
+}