Add new tests for alt, title, and attribute filtering

[jenlampton]

Add more simpletests.

This is a follow-up to CKEditor image captions break if you include a colon and Issue #4158: Title and alt attributes may be stripped if they contain a colon.

[jenlampton]

I've filed a PR that includes some of the tests from https://www.drupal.org/project/drupal/issues/2859006#comment-13786392.

There are two tests in the PR that are commented out because they test that data attributes are sanitized -- this is something Drupal is debating doing, but backdrop has chosen not to do. Depending on where Drupal lands on this, we may choose to reconsider.

[klonos]

Thanks @jenlampton ...the code looks good 👍 ...there's nothing that can be tested, since what was added is tests 🙂 ...the tests have not failed, so we can count that as a test I guess.

[ghost]

Requested some changes in PR.