E2E/DiffTest: fix line endings

As far as I can see, this test is about the display of the diff, not about the line endings, in which case, let's make the line endings explicit and not OS-dependent to allow this test to pass on Windows as well.

Author: jrfnl

tests/end-to-end/testdox/_files/DiffTest.php

@@ -9,16 +9,15 @@
  */
 namespace PHPUnit\TestFixture\TestDox;
 
-use const PHP_EOL;
 use PHPUnit\Framework\TestCase;
 
 final class DiffTest extends TestCase
 {
     public function testSomethingThatDoesNotWork(): void
     {
         $this->assertEquals(
-            'foo' . PHP_EOL . 'bar' . PHP_EOL . 'baz' . PHP_EOL,
-            'foo' . PHP_EOL . 'baz' . PHP_EOL . 'bar' . PHP_EOL,
+            "foo\nbar\nbaz\n",
+            "foo\nbaz\nbar\n",
         );
     }
 }