Add sizes to template_preprocess_image

Assuming test bot is fine with it.

1. +++ b/core/modules/system/src/Tests/Theme/ImageTest.php
   @@ -0,0 +1,49 @@
   +class ImageTest extends DrupalUnitTestBase {
   

   Once again, should be KernelTestBase.

2. +++ b/core/modules/system/src/Tests/Theme/ImageTest.php
   @@ -0,0 +1,49 @@
   +   * Tests that an image with the srcset and widths is output correctly.
   ...
   +    $this->assertRaw($expected, 'Correct output for image with srcset attribute and width descriptors.');
   

   It does not actually test srcset.

Also, suggestion from dawehner in IRC on the test itself (this would also apply to the srcset issue):

<dawehner> checking for exact HTML output is a bad idea
<dawehner> I would suggest to better parse the HTML and look at it
<dawehner> use $this->drupalSetRawContent() and then get the dom element and look at it
<dawehner> for example the order of the attributes could change easily which doesn't matter here at all

Since both this and the other patch are going to smack into each other, if you want to combine them into one, feel free.

AFAIK it uses the same principle as the image module for testing, so we need issues to update those tests as well, @dawehner is right that testing HTML is a bit lame.

New test, since xpath is not working (#2335057: Undefined method getUrl in AssertContentTrait::parse causes fatal error) is uses assertRaw.

Patch doesn't apply. Needs reroll.

This test looks much better, though. I thought I remember seeing something about calc possibly not working within a sizes attribute. It would be good to verify that before we use it in a test.

Assuming test bot will be happy, this is RTBC.

The issue to test the real DOM is still a valid point, but can be solved in a follow up.

For what it's worth, found out you can use calc within sizes. But we don't need that in the test, so this still looks good to go for me.

Committed bb3ee20 and pushed to 8.0.x. Thanks!

Minor: test comment says "test with multipliers", while this is not what the test does.

Also, would have been nice sugar to accept sizes as an array of strings, not requiring callers to do the implode themselves ?

Follow up created #2343657: Cleanup: Add sizes to template_preprocess_image