Refactor File and Image related image field creation logic into a new trait

Yes please! Some minor stuff:

1. index 0000000..3af9617
   --- /dev/null
   
   --- /dev/null
   +++ b/core/modules/image/src/Tests/ImageFieldCreationTrait.php
   

   We don't want to create new test related files in the src/Tests folder of a module because this is the old deprecated Simpletest location. Any test related stuff should be in tests/src.

2. +++ b/core/modules/image/src/Tests/ImageFieldCreationTrait.php
   @@ -0,0 +1,68 @@
   +  function createImageField($name, $type_name, $storage_settings = array(), $field_settings = array(), $widget_settings = array(), $formatter_settings = array(), $description = '') {
   

   visibility of the function is missing, I think this should be "protected", right?

3. +++ b/core/modules/image/src/Tests/ImageFieldTestBase.php
   index 0000000..d919377
   --- /dev/null
   
   --- /dev/null
   +++ b/core/modules/simpletest/src/TestFilesTrait.php
   

   I'm not happy that we create new stuff in the deprecated simpletest module, but since it calls simpletest_*() functions I guess this is ok for now.

This issue contains some bits from #2738567: Add test trait for drupalGetTestFiles and drupalCompareFile, so we maybe should postpone it for now?

@samuel.mortenson
Do you mind rerolling your patch with the trait from the other issue?

Well, apply the other patch locally and try to write the ImageFieldCreationTrait on top of it :)

The idea behind the issue is good.

Patch is clean. in that it does what is stated and nothing else.

My mistake

Trait 'Drupal\image\Tests\ImageFieldCreationTrait' not found in /var/www/html/core/modules/image/src/Tests/ImageFieldTestBase.php on line 25

I will fix this up.

@samuel.mortenson - Ah you are welcome.

My preferred home in the namespace tree was

1. Drupal\Tests\image\ImageFieldCreationTrait
2. Drupal\Tests\image\Traits\ImageFieldCreationTrait

but the PSR autoloader does not seems to be that flexible.

So I went with the next best

Drupal\Tests\image\Kernel\ImageFieldCreationTrait

If anyone has better suggestions ... my solution seems like a cludge!

Drupal\Tests\image\Kernel\ImageFieldCreationTrait

Well, maybe we should add an issue about it?

Beside from that though, this looks pretty nice

I'm wondering whether we should have dedicated tests for this new trait.

I have opened a side issue for a better home for the trait.

A minor nudge added an optional prefix to some @params in the new trait

Thank you @martin107

This blocks #2421427: Improve the UX of Quick Editing single-valued image fields.

As a simple code move in tests this is rc eligible. Committed and pushed 66741e3 to 8.3.x and 18da006 to 8.2.x. Thanks!