Forward port tests for image_style_deliver can create invalid headers

I didn't read the parent issue. I'm not sure I totally understand everything here, so take some of these review points with a dash of salt. 😉 Mostly nits, but a few points of substance:

1. +++ b/core/modules/image/tests/modules/image_module_test/image_module_test.module
   @@ -12,6 +12,10 @@ function image_module_test_file_download($uri) {
   +  if ($header_uri == $uri) {
   

   Should be ===

2. +++ b/core/modules/image/tests/src/Functional/ImageStylesPathAndUrlTest.php
   @@ -326,4 +326,23 @@ public function doImageStyleUrlAndPathTests($scheme, $clean_url = TRUE, $extra_s
   +   * Test that multiple same headers does not create a PHP notice.
   

   This doesn't follow our standards, and doesn't make a ton of English sense. How about:

   "Tests that a page with duplicate headers does not create a PHP notice."

   ? I'm not totally following what we're testing here. 😉

3. +++ b/core/modules/image/tests/src/Functional/ImageStylesPathAndUrlTest.php
   @@ -326,4 +326,23 @@ public function doImageStyleUrlAndPathTests($scheme, $clean_url = TRUE, $extra_s
   +  public function testImageContentTypeHeaders() {
   

   a. wants a : void return type.

   b. Is that really what we're testing here? The name doesn't seem to match the comment above (if I understand the comment above 😅).

4. +++ b/core/modules/image/tests/src/Functional/ImageStylesPathAndUrlTest.php
   @@ -326,4 +326,23 @@ public function doImageStyleUrlAndPathTests($scheme, $clean_url = TRUE, $extra_s
   +    $files = $this->drupalGetTestFiles('image');
   +    $file = array_shift($files);
   ...
   +    $this->assertSession()->responseHeaderEquals('Content-Type', 'image/png');
   

   Does anything ensure that the first $file we get from drupalGetTestFiles() is definitely going to be a .png? If that ever changes, isn't this test going to silently stop proving anything?

5. +++ b/core/modules/image/tests/src/Functional/ImageStylesPathAndUrlTest.php
   @@ -326,4 +326,23 @@ public function doImageStyleUrlAndPathTests($scheme, $clean_url = TRUE, $extra_s
   +    $file_system = \Drupal::service('file_system');
   

   Since we only use this once, do we need the local variable for it?

6. +++ b/core/modules/image/tests/src/Functional/ImageStylesPathAndUrlTest.php
   @@ -326,4 +326,23 @@ public function doImageStyleUrlAndPathTests($scheme, $clean_url = TRUE, $extra_s
   +    // Copy the test file to private folder.
   

   ... to the private folder

7. +++ b/core/modules/image/tests/src/Functional/ImageStylesPathAndUrlTest.php
   @@ -326,4 +326,23 @@ public function doImageStyleUrlAndPathTests($scheme, $clean_url = TRUE, $extra_s
   +    // Clean up.
   +    \Drupal::state()->delete('image.test_invalid_headers');
   

   Do we also want/need to remove the file from the private folder here?

Addressed points 1,2,3(a),5 and 6 from #23

Strangely running PHPCBF is making the patch fail. Adding the patch again.

Addressed points 1,2,3(a),5 and 6 from #23

Thanks for the interest

Shouldn't be moved to review until all the points are addressed. But helps that some of them have been!