E2E Tests - Adds more Highlight color visual tests

[geriux]

Adds two new visual tests to check the editor doesn't crash when using shorthand hex color codes with the highlight color feature:

• should set a shorthand hex highlight color

For this test it sets theme data with a color containing the #000 value.

• should render a mark tag with shorthand hex highlight color

This sets the initial content of the editor with a Paragraph block containing a mark tag (highlight formatting) with a #000 color.

This was addressed in Aztec Android https://github.com/wordpress-mobile/AztecEditor-Android/pull/1079 so to prevent future issues these tests were added.

To test all CI checks should pass.

PR submission checklist:

• [x] I have considered adding unit tests where possible.
• [x] I have considered if this change warrants user-facing release notes more info and have added them to RELEASE-NOTES.txt if necessary.