Use tree folder in ndjson project import spec fixture

What does this MR do?

Replace tree.tar.gz with extracted tree folder for 7 project import test fixtures under spec/fixtures/lib/gitlab/import_export:

• spec/fixtures/lib/gitlab/import_export/complex/tree.tar.gz
• spec/fixtures/lib/gitlab/import_export/group/tree.tar.gz
• spec/fixtures/lib/gitlab/import_export/invalid_json/tree.tar.gz
• spec/fixtures/lib/gitlab/import_export/light/tree.tar.gz
• spec/fixtures/lib/gitlab/import_export/milestone-iid/tree.tar.gz
• spec/fixtures/lib/gitlab/import_export/multi_pipeline_ref_one_external_pr/tree.tar.gz
• spec/fixtures/lib/gitlab/import_export/with_invalid_records/tree.tar.gz

Accordingly, update related Rspec test to remove extract/cleanup logic

Conformity

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

Closes #215867 (closed)