Set uploader to Pleroma.Uploaders.Local for failing tests

There were tests who were failing with

```
     The following arguments were given to IO.chardata_to_string/1:

         # 1
         nil

     Attempted function clauses (showing 2 out of 2):

         def chardata_to_string(string) when is_binary(string)
         def chardata_to_string(list) when is_list(list)
```

I noticed for several of them the problem was that the uploader was set to S3, while the rest of the S3 settings were nil.
The uploader itself didn't really matter much for the tests, but if it throws errors because it has settings that can't work, then the test fails.
I made changed the uploader to local first.

test/pleroma/web/activity_pub/activity_pub_test.exs

@@ -23,6 +23,8 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubTest do
   import Tesla.Mock
 
   setup do
+    clear_config([Pleroma.Upload, :uploader], Pleroma.Uploaders.Local)
+    clear_config([Pleroma.Uploaders.Local, :uploads], "uploads")
     mock(fn env -> apply(HttpRequestMock, :request, [env]) end)
     :ok
   end

test/pleroma/web/common_api_test.exs

@@ -27,6 +27,8 @@ defmodule Pleroma.Web.CommonAPITest do
   require Pleroma.Constants
 
   setup_all do
+    clear_config([Pleroma.Upload, :uploader], Pleroma.Uploaders.Local)
+    clear_config([Pleroma.Uploaders.Local, :uploads], "uploads")
     Tesla.Mock.mock_global(fn env -> apply(HttpRequestMock, :request, [env]) end)
     :ok
   end

test/pleroma/web/mastodon_api/controllers/media_controller_test.exs

@@ -13,6 +13,8 @@ defmodule Pleroma.Web.MastodonAPI.MediaControllerTest do
 
   describe "Upload media" do
     setup do: oauth_access(["write:media"])
+    setup do: clear_config([Pleroma.Upload, :uploader], Pleroma.Uploaders.Local)
+    setup do: clear_config([Pleroma.Uploaders.Local, :uploads], "uploads")
 
     setup do
       image = %Plug.Upload{

test/pleroma/web/mastodon_api/controllers/status_controller_test.exs

@@ -25,6 +25,8 @@ defmodule Pleroma.Web.MastodonAPI.StatusControllerTest do
   setup do: clear_config([:rich_media, :enabled])
   setup do: clear_config([:mrf, :policies])
   setup do: clear_config([:mrf_keyword, :reject])
+  setup do: clear_config([Pleroma.Upload, :uploader], Pleroma.Uploaders.Local)
+  setup do: clear_config([Pleroma.Uploaders.Local, :uploads], "uploads")
 
   describe "posting statuses" do
     setup do: oauth_access(["write:statuses"])

test/pleroma/web/pleroma_api/controllers/chat_controller_test.exs

@@ -106,6 +106,9 @@ defmodule Pleroma.Web.PleromaAPI.ChatControllerTest do
     end
 
     test "it works with an attachment", %{conn: conn, user: user} do
+      clear_config([Pleroma.Upload, :uploader], Pleroma.Uploaders.Local)
+      clear_config([Pleroma.Uploaders.Local, :uploads], "uploads")
+
       file = %Plug.Upload{
         content_type: "image/jpeg",
         path: Path.absname("test/fixtures/image.jpg"),