[tests] Only call MTLDevice.CreateSharedTexture macOS 10.14+. Fixes xamarin/maccore#1663. (#6180)

Fixes https://github.com/xamarin/maccore/issues/1663.

tests/monotouch-test/Metal/MTLDeviceTests.cs

@@ -110,6 +110,7 @@ namespace MonoTouchFixtures.Metal {
 			}
 
 #if __MACOS__
+			if (TestRuntime.CheckXcodeVersion (10, 0)) {
 				using (var descriptor = MTLTextureDescriptor.CreateTexture2DDescriptor (MTLPixelFormat.RGBA8Unorm, 64, 64, false)) {
 					descriptor.StorageMode = MTLStorageMode.Private;
 					using (var texture = device.CreateSharedTexture (descriptor)) {
@@ -120,6 +121,7 @@ namespace MonoTouchFixtures.Metal {
 							Assert.IsNotNull (texture, "CreateSharedTexture (MTLSharedTextureHandle): NonNull");
 					}
 				}
+			}
 #endif
 
 			using (var queue = device.CreateCommandQueue (10)) {