fix redis-benchmark's bug: check if clients are created successfully in idle mode (#10891)

my maxclients config:
```
redis-cli config get maxclients
1) "maxclients"
2) "4064"
```

Before this bug was fixed, creating 4065 clients appeared to be successful, but only 4064 were actually created```
```
./redis-benchmark -c 4065 -I
Creating 4065 idle connections and waiting forever (Ctrl+C when done)
cients: 4065
```

now :
```
./redis-benchmark -c 4065 -I
Creating 4065 idle connections and waiting forever (Ctrl+C when done)
Error from server: ERR max number of clients reached

./redis-benchmark -c 4064 -I
Creating 4064 idle connections and waiting forever (Ctrl+C when done)
clients: 4064

```

1 files changed, 9 insertions, 0 deletions

diff --git a/tests/integration/redis-benchmark.tcl b/tests/integration/redis-benchmark.tcl
index 6aa4c7882..ac894aed0 100644
--- a/tests/integration/redis-benchmark.tcl
+++ b/tests/integration/redis-benchmark.tcl
@@ -116,6 +116,15 @@ start_server {tags {"benchmark network external:skip"}} {
             # ensure the keyspace has the desired size
             assert_match  {50} [scan [regexp -inline {keys\=([\d]*)} [r info keyspace]] keys=%d]
         }
+        
+        test {benchmark: clients idle mode should return error when reached maxclients limit} {
+            set cmd [redisbenchmark $master_host $master_port "-c 2 -I"]
+            set original_maxclients [lindex [r config get maxclients] 1]
+            r config set maxclients 1
+            catch { exec {*}$cmd } error
+            assert_match "*ERR max number of clients reached*" $error
+            r config set maxclients $original_maxclients
+        }
 
         # tls specific tests
         if {$::tls} {