test: add more representation tests for perpare_outgoing

Port of commit 272799da6242dbf7387d2d42dfc98512cd7efd7e from
Akkoma PR 1018.

Changes from Akkoma commit:
- changed order of arguments in CommonAPI.(un)block, because Akkoma
  hasn't backported our change for the unified arg order yet

In particular this covers the case
e88f36f72b5317debafcc4209b91eb35ad8f0691 was meant to fix and

test/pleroma/web/activity_pub/transmogrifier_test.exs

@@ -611,6 +611,70 @@ defmodule Pleroma.Web.ActivityPub.TransmogrifierTest do
              } = prepared["object"]
     end
 
+    test "Correctly handles Undo activities" do
+      blocked = insert(:user)
+      blocker = insert(:user, local: true)
+
+      blocked_ap_id = blocked.ap_id
+      blocker_ap_id = blocker.ap_id
+
+      {:ok, %Activity{} = block_activity} = CommonAPI.block(blocked, blocker)
+      {:ok, %Activity{} = undo_activity} = CommonAPI.unblock(blocked, blocker)
+      {:ok, data} = Transmogrifier.prepare_outgoing(undo_activity.data)
+
+      block_ap_id = block_activity.data["id"]
+      assert is_binary(block_ap_id)
+
+      assert match?(
+               %{
+                 "@context" => [_ | _],
+                 "type" => "Undo",
+                 "id" => "http://localhost" <> _,
+                 "actor" => ^blocker_ap_id,
+                 "object" => ^block_ap_id,
+                 "to" => [^blocked_ap_id],
+                 "cc" => [],
+                 "bto" => [],
+                 "bcc" => []
+               },
+               data
+             )
+    end
+
+    test "Correctly handles EmojiReact activities" do
+      user = insert(:user, local: true)
+      note_activity = insert(:note_activity)
+
+      user_ap_id = user.ap_id
+      user_followers = user.follower_address
+      note_author = note_activity.data["actor"]
+      note_ap_id = note_activity.data["object"]
+
+      assert is_binary(note_author)
+      assert is_binary(note_ap_id)
+
+      {:ok, react_activity} = CommonAPI.react_with_emoji(note_activity.id, user, "🐈")
+      {:ok, data} = Transmogrifier.prepare_outgoing(react_activity.data)
+
+      assert match?(
+               %{
+                 "@context" => [_ | _],
+                 "type" => "EmojiReact",
+                 "actor" => ^user_ap_id,
+                 "to" => [^user_followers, ^note_author],
+                 "cc" => ["https://www.w3.org/ns/activitystreams#Public"],
+                 "bto" => [],
+                 "bcc" => [],
+                 "content" => "🐈",
+                 "context" => "2hu",
+                 "id" => "http://localhost" <> _,
+                 "object" => ^note_ap_id,
+                 "tag" => []
+               },
+               data
+             )
+    end
+
     test "it prepares a quote post" do
       user = insert(:user)