[backup] in test: call `sync` instead of wait for 50ms
[backup] in test: call `sync` instead of wait for 50ms

it takes a little while for `ls` to see new entries, and sleeping for 50ms is not that reliable way to get around it.

Closes: #5394
Approved by: lightmark

aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra