Fix image layout tests with newer PHPUnit versions

`assertSame` uses a stricter epsilon since version 9.5.14 (absolute
`PHP_FLOAT_EPSILON`), see sebastianbergmann/phpunit#4874.
Just rely on the recently introduced `Helpers::lengthEqual` for length
comparison.

tests/LayoutTest/ImageTest.php

@@ -3,6 +3,7 @@
 
 use Dompdf\Dompdf;
 use Dompdf\FrameDecorator\AbstractFrameDecorator;
+use Dompdf\Helpers;
 use Dompdf\Options;
 use Dompdf\Tests\TestCase;
 
@@ -216,7 +217,13 @@ public function testImageDimensions(
         );
         $dompdf->render();
 
-        $this->assertSame($expectedWidth, $width);
-        $this->assertSame($expectedHeight, $height);
+        $this->assertTrue(
+            Helpers::lengthEqual($expectedWidth, $width),
+            "Failed asserting that $width is equal to $expectedWidth."
+        );
+        $this->assertTrue(
+            Helpers::lengthEqual($expectedHeight, $height),
+            "Failed asserting that $height is equal to $expectedHeight."
+        );
     }
 }