Add more MusicStore and Identity tests

Covering all the queries MusicStore does, and also updating the tests to properly use store-generated values.

Also, added the ModelSnapshots generated by the project templates for 2.1, 2.2, and 3.0, for SQL Server and SQLite. They all diff correctly against the current code showing no differences in the generated schema.

Part of #15662 and #11838

Note that currently the ASP.NET 3.0 templates still contain migrations generated by 2.2. See dotnet/aspnetcore#12168

test/EFCore.Relational.Specification.Tests/MigrationsTestBase.cs

@@ -357,6 +357,26 @@ protected virtual Task ExecuteAsync(IServiceProvider services, Action<MigrationB
         [ConditionalFact]
         public abstract void Can_diff_against_2_2_model();
 
+        [ConditionalFact]
+        public abstract void Can_diff_against_3_0_ASP_NET_Identity_model();
+
+        [ConditionalFact]
+        public abstract void Can_diff_against_2_2_ASP_NET_Identity_model();
+
+        [ConditionalFact]
+        public abstract void Can_diff_against_2_1_ASP_NET_Identity_model();
+
+        protected virtual void DiffSnapshot(ModelSnapshot snapshot, DbContext context)
+        {
+            var sourceModel = snapshot.Model;
+            var targetModel = context.Model;
+
+            var modelDiffer = context.GetService<IMigrationsModelDiffer>();
+            var operations = modelDiffer.GetDifferences(sourceModel, targetModel);
+
+            Assert.Equal(0, operations.Count);
+        }
+
         protected virtual void BuildFirstMigration(MigrationBuilder migrationBuilder)
         {
             migrationBuilder.CreateTable(