Use tree folder instead of tree.tar.gz in ndjson project import test fixture

This is a follow-up issue of ndjson project import MR(!27206 (merged)).

In MR !27206 (merged), we created test fixtures for ndjson project import:

• 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

For each of these tree.tar.gz, the Rspec test needs to extract it into a tree folder before running the test. The only reason we archived tree into tree.tar.gz, was to reduce the noise in MR review, because the tree folder can contain many files.

For the long term, it is more convenient to use the untarred tree folder directly:

• it removes the complexity to extract and cleanup for the test. Using tarballs requires additional test setup(extract tree folder from tree.tar.gz in before do) and extra cleanup(delete tree folder in after do). This introduces unnecessary complexity to maintain spec. This had caused tricky flaky pipeline issues: #213843 (closed) and #213793 (closed)
• it allows git to better track individual file change, thus make review easier

This issue is to replace these 7 tree.tar.gz(listed above) with tree folder.