## Describe your changes
test_on_demand_download is flaky because not waiting until created image
layer is transferred to S3.
test_tenants_with_remote_storage just leaves garbage at the end of
overwritten file.
Right solution for test_on_demand_download is to add some API call to
wait completion of synchronization with S3 (not just based on last
record LSN). But right now it is solved using sleep.
## Issue ticket number and link
#3209
## Checklist before requesting a review
- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.
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