test: Attempt to de-flake mistty-test-forbid-edit-map.

Sometimes (execute-kbd-macro "<right>") doesn't seems to register in
mistty-test-forbid-edit-map. This change assumes that the problem comes
from a race condition due to <right> and RET being sent in different
ways and switches to sending them both with execute-kbd-macro.

test/mistty-test.el

@@ -3105,8 +3105,7 @@
 
       ;; "echo third", the first option is selected by default.
       ;; select the second option (echo second) and accept it.
-      (execute-kbd-macro (kbd "<right>"))
-      (mistty-send-command)
+      (execute-kbd-macro (kbd "<right> RET"))
       (mistty-wait-for-output
        :test (lambda ()
                (save-excursion