Add the test cases for ValidationRuleSet

Author: xuzhg

src/Microsoft.OpenApi/Validations/ValidationRule.cs

@@ -5,7 +5,7 @@
 using System.Diagnostics;
 using Microsoft.OpenApi.Interfaces;
 
-namespace Microsoft.OpenApi.Validations.Rules
+namespace Microsoft.OpenApi.Validations
 {
     /// <summary>
     /// Class containing validation rule logic.

src/Microsoft.OpenApi/Validations/ValidationRuleSet.cs

@@ -8,8 +8,9 @@
 using System.Collections.Generic;
 using Microsoft.OpenApi.Exceptions;
 using Microsoft.OpenApi.Properties;
+using Microsoft.OpenApi.Validations.Rules;
 
-namespace Microsoft.OpenApi.Validations.Rules
+namespace Microsoft.OpenApi.Validations
 {
     /// <summary>
     /// The rule set of the validation.

test/Microsoft.OpenApi.Tests/Validations/ValidationRuleSetTests.cs

@@ -0,0 +1,40 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT license.
+
+using System.Linq;
+using Xunit;
+
+namespace Microsoft.OpenApi.Validations.Tests
+{
+    public class ValidationRuleSetTests
+    {
+        [Fact]
+        public void DefaultRuleSetReturnsTheCorrectRules()
+        {
+            // Arrange
+            var ruleSet = new ValidationRuleSet();
+
+            // Act
+            var rules = ruleSet.Rules;
+
+            // Assert
+            Assert.NotNull(rules);
+            Assert.Empty(rules);
+        }
+
+        [Fact]
+        public void DefaultRuleSetPropertyReturnsTheCorrectRules()
+        {
+            // Arrange & Act
+            var ruleSet = ValidationRuleSet.DefaultRuleSet;
+            Assert.NotNull(ruleSet); // guard
+
+            var rules = ruleSet.Rules;
+
+            // Assert
+            Assert.NotNull(rules);
+            Assert.NotEmpty(rules);
+            Assert.Equal(13, rules.ToList().Count); // please update the number if you add new rule.
+        }
+    }
+}