Unit tests cleanup

[Tabrisrp]

Description

Cleanup the unit tests suite:

• Add missing fixtures file
• Remove no longer used/empty tests files
• Fix deprecation warning

Sub-task of #6607

Type of change

• Bug fix (non-breaking change which fixes an issue).

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for 2b088a31	✅ ∅ (target: 50.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (2b088a3)	Report Missing	Report Missing	Report Missing
Head commit (2d8f18a)	37183	14396	38.72%

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 (#6622)	0	0	∅ (not applicable)

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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Footnotes

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[jeawhanlee]

Does this fix cover PHP 8.1, 8.2 & 8.3 as there's still deprecated warning for it:
WP latest with PHP 8.3 on ubuntu-latest.

[Tabrisrp]

Good catch, I didn't check those ones. I removed the invalid annotations.

For the warnings created by str_replace(), I'll open an issue on the library to fix this upstream