Improve flickering test on CI

pgconn/pgconn_test.go

@@ -3632,6 +3632,7 @@ func TestCancelRequestContextWatcherHandler(t *testing.T) {
 					ctx, cancel := context.WithTimeout(context.Background(), 4*time.Millisecond)
 					defer cancel()
 					pgConn.Exec(ctx, "select 1, pg_sleep(0.010)").ReadAll()
+					time.Sleep(100 * time.Millisecond) // ensure a cancel request that was a little late doesn't interrupt ensureConnValid.
 					ensureConnValid(t, pgConn)
 				}()
 			}