add test for custom alias page

tests/dashboard/test_custom_alias.py

@@ -0,0 +1,22 @@
+from flask import url_for
+
+from app.config import EMAIL_DOMAIN
+from app.extensions import db
+from app.utils import random_word
+from tests.utils import login
+
+
+def test_add_alias_success(flask_client):
+    login(flask_client)
+    db.session.commit()
+
+    word = random_word()
+
+    r = flask_client.post(
+        url_for("dashboard.custom_alias"),
+        data={"prefix": "prefix", "suffix": f".{word}@{EMAIL_DOMAIN}"},
+        follow_redirects=True,
+    )
+
+    assert r.status_code == 200
+    assert f"Alias prefix.{word}@{EMAIL_DOMAIN} has been created" in str(r.data)