Try to solve valgrind CI test error with client-eviction test (#11822)

The test sporadically failed with valgrind trying to match
`no client named obuf-client1 found*`
in the log it looks like `obuf-client1` was indeed dropped,
so i'm guessing it's because CLIENT LIST was processed first.

tests/unit/client-eviction.tcl

@@ -353,6 +353,7 @@ start_server {} {
         $rr3 get k
         $rr3 flush
         exec kill -SIGCONT $server_pid
+        r ping ;# make sure a full event loop cycle is processed before issuing CLIENT LIST
 
         # Validate obuf-clients were disconnected (because of obuf limit)
         catch {client_field obuf-client1 name} e