Add internal links to LinkItemUrlValidationTest

Create draft MR, need to decide what other internal links to test.

Add internal links.
Improved error message. Provide violation message.

Think if we are going to refactor this why not use a dataprovider?

Think if we are going to refactor this why not use a dataprovider?

I'd seen the docblock of LinkSeparateFormatterDisplayTest::testLinkSeparateFormatter() that says "Run tests without dataProvider to improve speed." The issue for which that test was created noted that not using a data provider was a considerable performance increase. So that made me curious, what would be the performance impact of running these tests with a data provider?

As they are now, the tests run in 3-5 seconds on my local environment. I refactored them to use data providers instead. The external link tests alone took 1min 17sec. The internal link tests didn't run because the provider function uses $this which isn't allowed in a static function. So that would take further refactoring. But I think it's clear that using a provider is less preferable without doing that extra work.

I left a couple of comments on the MR.