[Pixel Diff Ash] Move&rename ash pixel test code

This CL moves the ash pixel test code located under ash/test to a
separate directory.

In addition, this CL:

1. Renames `AshPixelDiffTestHelper` to `AshPixelDiffer`
2. Renames `AshTestUiStabilizer` to `AshPixelTestHelper`

This CL should not introduce any noticeable changes.

Bug: 1347171
Change-Id: I4300b657019f0e36798d02f5e4e2c5e289d8869f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3976309
Commit-Queue: Andrew Xu <[email protected]>
Reviewed-by: Xiyuan Xia <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1063480}
diff --git a/ash/fullscreen_pixeltest.cc b/ash/fullscreen_pixeltest.cc
index 067c392..91392ce8 100644
--- a/ash/fullscreen_pixeltest.cc
+++ b/ash/fullscreen_pixeltest.cc
@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "ash/test/ash_pixel_diff_test_helper.h"
-#include "ash/test/ash_pixel_test_init_params.h"
 #include "ash/test/ash_test_base.h"
+#include "ash/test/pixel/ash_pixel_differ.h"
+#include "ash/test/pixel/ash_pixel_test_init_params.h"
 
 namespace ash {