Update LocalAppContext DisableCaching and XmlSchemaSet Tests to fix failure (dotnet/corefx#14778)

* Update LocalAppContext DisableCaching and XmlSchemaSet Tests


Commit migrated from https://github.com/dotnet/corefx/commit/05ae65fd6b51f865750e8d400c60511fcdbbbfb5

src/libraries/Common/src/System/LocalAppContext.cs

@@ -8,14 +8,9 @@ namespace System
 {
     internal partial class LocalAppContext
     {
-        static LocalAppContext()
-        {
-            bool isEnabled;
-            if (AppContext.TryGetSwitch(@"TestSwitch.LocalAppContext.DisableCaching", out isEnabled))
-            {
-                DisableCaching = isEnabled;
-            }
-        }
+        private static bool s_isDisableCachingInitialized;
+        private static bool s_disableCaching;
+        private static readonly object s_syncObject = new object();
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static bool GetCachedSwitchValue(string switchName, ref int switchValue)
@@ -40,6 +35,29 @@ private static bool GetCachedSwitchValueInternal(string switchName, ref int swit
             return isSwitchEnabled;
         }
 
-        private static bool DisableCaching { get; set; }
+        private static bool DisableCaching 
+        { 
+            get 
+            {
+                if (!s_isDisableCachingInitialized)
+                {
+                    lock (s_syncObject)
+                    {
+                        if (!s_isDisableCachingInitialized)
+                        {
+                            bool isEnabled;
+                            if (AppContext.TryGetSwitch(@"TestSwitch.LocalAppContext.DisableCaching", out isEnabled))
+                            {
+                                s_disableCaching = isEnabled;
+                            }
+
+                            s_isDisableCachingInitialized = true;
+                        }
+                    }
+                }
+
+                return s_disableCaching;
+            }
+        }
     }
 }

src/libraries/System.Private.Xml/tests/XmlSchema/XmlSchemaSet/TC_SchemaSet.cs

@@ -8,6 +8,11 @@ namespace System.Xml.Tests
 {
     public class TestData
     {
+        static TestData()
+        {
+            AppContext.SetSwitch("TestSwitch.LocalAppContext.DisableCaching", true);
+        }
+        
         internal static string _Root = Path.Combine("TestFiles", "TestData");
         internal static string StandardPath = Path.Combine("TestFiles", "StandardTests");
         internal static string _FileXSD1 = Path.Combine(_Root, "schema1.xsd");

src/libraries/System.Private.Xml/tests/XmlSchema/XmlSchemaSet/TC_SchemaSet_XmlResolver.cs

@@ -16,9 +16,6 @@ public class TC_SchemaSet_XmlResolver
 
         public TC_SchemaSet_XmlResolver(ITestOutputHelper output)
         {
-            // Make sure that we don't cache the value of the switch to enable testing
-            AppContext.SetSwitch("TestSwitch.LocalAppContext.DisableCaching", true);
-
             _output = output;
         }
 
@@ -97,7 +94,6 @@ public void v3()
         }
 
         //[Variation(Desc = "v4 - schema(Local)->schema(Local)", Priority = 1)]
-        [ActiveIssue(14064)]
         [Fact]
         public void v4()
         {
@@ -113,7 +109,6 @@ public void v4()
         }
 
         //[Variation(Desc = "v5 - schema(Local)->schema(Local)->schema(Local)", Priority = 1)]
-        [ActiveIssue(14064)]
         [Fact]
         public void v5()
         {