Skip the upload-path escaping test on Windows

`testFieldEscapesPathUrl` uploads a file whose client name carries a quote, to show that an
unrestricted field carries it into the attachment URL. Windows does not allow a quote in a
file name, so the upload is refused there and the URL the test asserts on cannot arise at
all. Skipped with that reason rather than weakened: `testFieldEscapesThumbUrl` already
covers the same partials from a stored record on every platform.

Author: LukeTowers

modules/backend/tests/formwidgets/FileUploadEscapingTest.php

@@ -135,6 +135,14 @@ public function testFieldEscapesThumbUrl(): void
      */
     public function testFieldEscapesPathUrl(): void
     {
+        if (DIRECTORY_SEPARATOR === '\\') {
+            $this->markTestSkipped(
+                'Windows rejects a file name containing a quote, so this upload cannot be stored there and '
+                . 'the URL it asserts on cannot arise. testFieldEscapesThumbUrl covers the same partials '
+                . 'from a stored record on every platform.'
+            );
+        }
+
         $single = $this->upload('avatar', self::UNSAFE_EXTENSION, ['mode' => 'file']);
 
         $this->assertStringContainsString(