ci: add a small delay between failed test retries. bump retries for channel_select_6_test.v to 8

cmd/tools/modules/testing/common.v

@@ -358,6 +358,7 @@ fn worker_trunner(mut p pool.PoolProcessor, idx int, thread_id int) voidptr {
 						goto test_passed_system
 					}
 				}
+				time.sleep(500 * time.millisecond)
 			}
 			ts.failed = true
 			ts.benchmark.fail()

vlib/sync/channel_select_6_test.v

@@ -1,4 +1,4 @@
-// vtest retry: 4
+// vtest retry: 8
 
 // This test case runs concurrent 3 instances of `do_select` that
 // communicate with 6 other threads doing send and receive operations.