Update and add more test coverage for `no-default-alt-text`

[khiga8]

In https://github.com/github/markdownlint-github/pull/83, we refactored the rule so we don't rely entirely on Regex for parsing, which was causing the rule to flag images within code blocks. (Edit: Like https://github.com/github/markdownlint-github/pull/84#issuecomment-1750683661)

This is a follow-up to that change. I noticed the rule was not correctly flagging the case where there are two images like so:

<img alt="image"/><img alt="image" />

This PR makes a few updates:

• Added detail to include the alt that is being flagged.
• Add test coverage and support for when there are multiple image tags in one line.
• Right now, the test helper doesn't account for how there can be multiple violations in one line. I slightly refactored it so it does, and made subsequent updates in files.

[github-actions[bot]]

Uh oh! @khiga8, the image you shared is missing helpful alt text. Check your pull request body.

Alt text is an invisible description that helps screen readers describe images to blind or low-vision users. If you are using markdown to display images, add your alt text inside the brackets of the markdown image.

Learn more about alt text at Basic writing and formatting syntax: images on GitHub Docs.

[github-actions[bot]]

Uh oh! @khiga8, the image you shared is missing helpful alt text. Check your pull request body.

Alt text is an invisible description that helps screen readers describe images to blind or low-vision users. If you are using markdown to display images, add your alt text inside the brackets of the markdown image.

Learn more about alt text at Basic writing and formatting syntax: images on GitHub Docs.

[github-actions[bot]]

Uh oh! @khiga8, the image you shared is missing helpful alt text. Check your pull request body.

Alt text is an invisible description that helps screen readers describe images to blind or low-vision users. If you are using markdown to display images, add your alt text inside the brackets of the markdown image.

Learn more about alt text at Basic writing and formatting syntax: images on GitHub Docs.

[github-actions[bot]]

Uh oh! @khiga8, the image you shared is missing helpful alt text. Check your pull request body.

Alt text is an invisible description that helps screen readers describe images to blind or low-vision users. If you are using markdown to display images, add your alt text inside the brackets of the markdown image.

Learn more about alt text at Basic writing and formatting syntax: images on GitHub Docs.

[github-actions[bot]]

Uh oh! @khiga8, the image you shared is missing helpful alt text. Check your pull request body.

Alt text is an invisible description that helps screen readers describe images to blind or low-vision users. If you are using markdown to display images, add your alt text inside the brackets of the markdown image.

Learn more about alt text at Basic writing and formatting syntax: images on GitHub Docs.

[github-actions[bot]]

Uh oh! @khiga8, the image you shared is missing helpful alt text. Check your pull request body.

Alt text is an invisible description that helps screen readers describe images to blind or low-vision users. If you are using markdown to display images, add your alt text inside the brackets of the markdown image.

Learn more about alt text at Basic writing and formatting syntax: images on GitHub Docs.