Add unit test for HasRegistrySupport()

The use of the function in RegistryProvider cannot be tested without a
mock of a PowerShell cmdlet context.

src/ps_test/test_CorePsPlatform.cs

@@ -30,5 +30,10 @@ namespace PSTests
         {
             Assert.False(Platform.HasDriveAutoMounting());
         }
+
+        public static void TestHasRegistrySupport()
+        {
+            Assert.False(Platform.HasRegistrySupport());
+        }
     }
 }