Fix unstable test: replication with parallel clients writing in different DBs (#11782)

Failure happens in FreeBSD daily:
```
*** [err]: Test replication with parallel clients writing in different DBs in tests/integration/replication-4.tcl
Expected [::redis::redisHandle2 dbsize] > 0 (context: type eval line 19 cmd {assert {[$master dbsize] > 0}} proc ::test)
```

The test is failing because db 9 has no data (default db), and
according to the log, we can see that db 9 does not have a key:
```
 ### Starting test Test replication with parallel clients writing in different DBs in tests/integration/replication-4.tcl
3338:S 03 Feb 2023 00:15:18.723 - DB 11: 1 keys (0 volatile) in 4 slots HT.
3338:S 03 Feb 2023 00:15:18.723 - DB 12: 141 keys (0 volatile) in 256 slots HT.
```

We use `wait_for_condition` to ensure that parallel clients have
written data before calling stop_bg_complex_data. At the same time,
`wait_for_condition` is also used to remove the above `after 1000`,
which can save time in most cases.