Fix race in temp rdb delete shutdown test (#11840)

I saw this error once, in the FreeBSD Daily CI:
```
*** [err]: Temp rdb will be deleted if we use bg_unlink when shutdown in tests/unit/shutdown.tcl
Expected [file exists /xxx/temp-10336.rdb] (context: type eval line 15 cmd {assert {[file exists $temp_rdb]}} proc ::test)
```

The log shows that bgsave was executed, and it was successfully executed in the end:
```
Starting test Temp rdb will be deleted if we use bg_unlink when shutdown in tests/unit/shutdown.tcl
10251:M 22 Feb 2023 11:37:25.441 * Background saving started by pid 10336
10336:C 22 Feb 2023 11:37:27.949 * DB saved on disk
10336:C 22 Feb 2023 11:37:27.949 * Fork CoW for RDB: current 0 MB, peak 0 MB, average 0 MB
10251:M 22 Feb 2023 11:37:28.060 * Background saving terminated with success
```

There may be two reasons:
1. The child process has been created, but it has not created
   the temp rdb file yet, so [file exists $temp_rdb] check failed.
2. The child process bgsave has been executed successfully and the
   temp file has been deleted, so [file exists $temp_rdb] check failed.

From the logs pint, it should be the case 2, case 1 is too extreme,
set rdb-key-save-delay to a higher value to ensure bgsave does not
succeed early to avoid this case.

0 files changed, 0 insertions, 0 deletions