:chore: Add scenario and step for relative lcp image template -- #85

[Khadreal]

Description

Add scenario to test for lcp image with relative url that should have fetchpriority attribute

Fixes #85

Documentation

User documentation

Explain how this code impacts users.

Technical documentation

Explain how this code works. Diagram & drawings are welcomed.

Type of change

• [x] Enhancement (non-breaking change which improves an existing functionality).

New dependencies

List any new dependencies that are required for this change.

Risks

List possible performance & security issues or risks, and explain how they have been mitigated.

Checklists

Feature validation

• [x] I validated all the Acceptance Criteria. If possible, provide sreenshots or videos.
• [x] I triggered all changed lines of code at least once without new errors/warnings/notices.
• [x] I implemented built-in tests to cover the new/changed code.

Code style

• [x] I wrote self-explanatory code about what it does.
• [x] I named variables and functions explicitely.
• [x] I did not introduce unecessary complexity.

[jeawhanlee]

After CR is done, We can merge when this is merged to trunk