Resolved image file URLs invalid in kernel tests

Overview

Discovered in #3530351: Decouple image+video (URI) shape matching from specific image+video file types/extensions. This bug makes that MR harder to understand.

In HEAD, any image field instance's values populates a component instance with either its url property or its src_with_alternate_widths property. Both are computed from the value field property. The results are:

• value = public://test.png
• url = /vfs://root/sites/simpletest/91669142/files/test.png
• src_with_alternate_widths = /vfs://root/sites/simpletest/91669142/files/test.png?alternateWidths=/vfs%3A//root/sites/simpletest/91669142/files/styles/xb_parametrized_width--%7Bwidth%7D/public/test.png.webp%3Fitok%3DFEbY0Wjq

🐛 The bug: neither of those latter 2 are valid URLs!

Now, in principle the SDC JSON Schema validation should've complained about that. But due to a bug in the regex (JSON Schema pattern) of json-schema-definitions://experience_builder.module/image-uri, which turns out to have been too broad/liberal in what it accepts. 😬 That's part of what #3530351: Decouple image+video (URI) shape matching from specific image+video file types/extensions is fixing, but in order to keep the scope/MR clear there, this has been lifted out of there.

Proposed resolution

Ensure /vfs://root/sites/simpletest/91669142/files/test.png gets rewritten to a valid URL.

Core has prior art for this: \Drupal\Tests\system\Kernel\Theme\TwigIncludeTest::setUpFilesystem(). Use that.

User interface changes

None.