Test: fix slowlog test false positive.

In fast systems "SLOWLOG RESET" is fast enough to don't be logged even
when the time limit is "1" sometimes. Leading to false positives such
as:

[err]: SLOWLOG - can be disabled in tests/unit/slowlog.tcl
Expected '1' to be equal to '0'

tests/unit/slowlog.tcl

@@ -80,9 +80,11 @@ start_server {tags {"slowlog"} overrides {slowlog-log-slower-than 1000000}} {
     }
 
     test {SLOWLOG - can be disabled} {
+        r config set slowlog-max-len 1
         r config set slowlog-log-slower-than 1
         r slowlog reset
-        assert_equal [r slowlog len] 1 
+        r debug sleep 0.2
+        assert_equal [r slowlog len] 1
         r config set slowlog-log-slower-than -1
         r slowlog reset
         r debug sleep 0.2