Closes #6728 Largest element with a background-image that is not kept

[Khadreal]

Description

Fixed the issue with LCP when background-image has gradient

Fixes wp-media/wp-rocket#6728

Type of change

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

Detailed scenario

Check scenario here

Technical description

Improve the beaconLCP script to capture valid image after background-image gradient.

Documentation

Mandatory Checklist

Code validation

• I validated all the Acceptance Criteria. If possible, provide screenshots or videos.
• I triggered all changed lines of code at least once without new errors/warnings/notices.

Code style

• I wrote a self-explanatory code about what it does.
• I protected entry points against unexpected inputs.
• I did not introduce unnecessary complexity.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +6.70% (target: -1.00%)	❌ 42.86% (target: 50.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (78ca211)	578	349	60.38%
Head commit (7a343c5)	641 (+63)	430 (+81)	67.08% (+6.70%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#33)	7	3	42.86%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[jeawhanlee]

Good Job here Sir
Can we also update related test case?

[piotrbak]

@Khadreal Is this one still in progress?

[Khadreal]

@Khadreal Is this one still in progress?

Nope

[hanna-meda]

@Khadreal, thank you for this PR.
Finished exploratory and also executed TP and all looks good on the test env.
Tests Results report below:
testrail-report-647.pdf