Tests: avoid short reads on redis-cli output. (#9301)

In some cases large replies on slow systems may only be partially read
by the test suite, resulting with parsing errors.

This fix is still timing sensitive but should greatly reduce the chances
of this happening.

tests/integration/redis-cli.tcl

@@ -31,13 +31,25 @@ start_server {tags {"cli"}} {
     }
 
     proc read_cli {fd} {
-        set buf [read $fd]
-        while {[string length $buf] == 0} {
-            # wait some time and try again
+        set ret [read $fd]
+        while {[string length $ret] == 0} {
             after 10
-            set buf [read $fd]
+            set ret [read $fd]
         }
-        set _ $buf
+
+        # We may have a short read, try to read some more.
+        set empty_reads 0
+        while {$empty_reads < 5} {
+            set buf [read $fd]
+            if {[string length $buf] == 0} {
+                after 10
+                incr empty_reads
+            } else {
+                append ret $buf
+                set empty_reads 0
+            }
+        }
+        return $ret
     }
 
     proc write_cli {fd buf} {