add test for guild nickname setting

tests/test_guild.py

@@ -78,3 +78,27 @@ async def test_guild_create(test_cli_user):
     resp = await test_cli_user.delete(f"/api/v6/guilds/{guild_id}")
 
     assert resp.status_code == 204
+
+
+@pytest.mark.asyncio
+async def test_guild_nickname(test_cli_user):
+    guild = await test_cli_user.create_guild()
+
+    NEW_NICKNAME = "my awesome nickname"
+    # stage 1: create
+    resp = await test_cli_user.patch(
+        f"/api/v6/guilds/{guild.id}/members/@me/nick",
+        json={"nick": NEW_NICKNAME},
+    )
+
+    assert resp.status_code == 200
+    assert (await resp.data).decode() == NEW_NICKNAME
+
+    # stage 2: test
+    resp = await test_cli_user.get(f"/api/v6/guilds/{guild.id}")
+
+    assert resp.status_code == 200
+    fetched_guild = await resp.json
+
+    assert fetched_guild["id"] == str(guild.id)
+    assert fetched_guild["members"][0]["nick"] == NEW_NICKNAME