Improve project Import coverage

What does this MR do?

Add more coverage into the existing Import test: expand JSON fixture we use and relevant test examples.

Partially resolves: #39536 (closed)

I will add other relations in the next MR to avoid blocks. In case adding them will be tricky, these specs could be in master already serving for the future refactoring.

At the moment of this MR being opened, my prototype "coverage checker" (the code in the end of !22251 (diffs)) tells that we need also to cover:

"metrics", 
"metrics_setting", 
"prometheus_metrics", 
"push_event_payload", 
"service_desk_setting", 
"unprotect_access_levels"

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

N/A

Security

N/A

Related to #39536 (closed)