Resolves#2212.
- use `wait_for_last_flush_lsn` in `test_timeline_physical_size_*` tests
## Context
Need to wait for the pageserver to catch up with the compute's last flush LSN because during the timeline physical size API call, it's possible that there are running `LayerFlushThread` threads. These threads flush new layers into disk and hence update the physical size. This results in a mismatch between the physical size reported by the API and the actual physical size on disk.
### Note
The `LayerFlushThread` threads are processed **concurrently**, so it's possible that the above error still persists even with this patch. However, making the tests wait to finish processing all the WALs (not flushing) before calculating the physical size should help reduce the "flakiness" significantly
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