Media: Improve and fix rotate/flip image tests.

Fix off-by-one error in pixel color checks for rotate and flip image tests. Change to using PNG with single pixel to ensure that errors are caught in the future, rather than lost in JPEG noise.

Props Fuegas, mikeschroder.
Merges [45067] to the 5.0 branch.
See #46073, #48301.

git-svn-id: https://develop.svn.wordpress.org/branches/5.0@49518 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

tests/phpunit/data/images/one-blue-pixel-100x100.png

@@ -420,15 +420,15 @@ public function test_crop() {
 	 * Test rotating an image 180 deg
 	 */
 	public function test_rotate() {
-		$file = DIR_TESTDATA . '/images/gradient-square.jpg';
+		$file = DIR_TESTDATA . '/images/one-blue-pixel-100x100.png';
 
 		$imagick_image_editor = new WP_Image_Editor_Imagick( $file );
 		$imagick_image_editor->load();
 
 		$property = new ReflectionProperty( $imagick_image_editor, 'image' );
 		$property->setAccessible( true );
 
-		$color_top_left = $property->getValue( $imagick_image_editor )->getImagePixelColor( 1, 1 )->getColor();
+		$color_top_left = $property->getValue( $imagick_image_editor )->getImagePixelColor( 0, 0 )->getColor();
 
 		$imagick_image_editor->rotate( 180 );
 
@@ -439,15 +439,15 @@ public function test_rotate() {
 	 * Test flipping an image
 	 */
 	public function test_flip() {
-		$file = DIR_TESTDATA . '/images/gradient-square.jpg';
+		$file = DIR_TESTDATA . '/images/one-blue-pixel-100x100.png';
 
 		$imagick_image_editor = new WP_Image_Editor_Imagick( $file );
 		$imagick_image_editor->load();
 
 		$property = new ReflectionProperty( $imagick_image_editor, 'image' );
 		$property->setAccessible( true );
 
-		$color_top_left = $property->getValue( $imagick_image_editor )->getImagePixelColor( 1, 1 )->getColor();
+		$color_top_left = $property->getValue( $imagick_image_editor )->getImagePixelColor( 0, 0 )->getColor();
 
 		$imagick_image_editor->flip( true, false );