test_empty_branch_remote_storage_upload_on_restart: rare flakyness

[koivunej]

With local_fs: https://neon-github-public-dev.s3.amazonaws.com/reports/pr-5022/5891420105/index.html#suites/837740b64a53e769572c4ed7b7a7eeeb/e703b4b5c28e29eb

[alexanderlaw]

The last commit I could reproduce (by running multiple test instances concurrently) this failure on is cd449d6~1.
cd449d6 replaced that failure-triggering assert (assert local_metadata.is_file()) with assert timeline_dir.is_dir() and I could not get a test failure on that commit using the same testing procedure.

Maybe this issue should be closed as resolved by #6769.