Fix timing issue in slowlog redact test (#10614)

* Fix timing issue in slowlog redact test

This test failed once in my daily CI (test-sanitizer-address (clang))
```
*** [err]: SLOWLOG - Some commands can redact sensitive fields in tests/unit/slowlog.tcl
Expected 'migrate 127.0.0.1 25649 key 9 5000 AUTH2 (redacted) (redacted)' to match '* key 9 5000 AUTH (redacted)' (context: type eval line 12 cmd {assert_match {* key 9 5000 AUTH (redacted)} [lindex [lindex [r slowlog get] 1] 3]} proc ::test)
```

The reason is that with slowlog-log-slower-than 10000,
slowlog get will have a chance to exceed 10ms.

Change slowlog-log-slower-than from 10000 to -1, disable it.
Also handles a same potentially problematic test above.
This is actually the same timing issue as #10432.

But also avoid repeated calls to `SLOWLOG GET`