Fix flaky PTTL time to live in milliseconds test on slow machines (#11651)

This test failed in FreeBSD:
```
*** [err]: PTTL returns time to live in milliseconds in tests/unit/expire.tcl
Expected 836 > 900 && 836 <= 1000 (context: type eval line 5 cmd {assert {$ttl > 900 && $ttl <= 1000}} proc ::test)
```

On some slow machines, sometimes the test take close to 200ms
to finish. We only set aside 100ms, so that caused the failure.
Since the failure was around 800, change the condition to be >500.

tests/unit/expire.tcl

@@ -151,7 +151,7 @@ start_server {tags {"expire"}} {
         r del x
         r setex x 1 somevalue
         set ttl [r pttl x]
-        assert {$ttl > 900 && $ttl <= 1000}
+        assert {$ttl > 500 && $ttl <= 1000}
     }
 
     test {TTL / PTTL / EXPIRETIME / PEXPIRETIME return -1 if key has no expire} {