MM-42201: Enforce group is not the same as username. (#19660)

* MM-42201: Enforce group is not the same as username.

* MM-42201: Adjust test for change to translation id.

app/user_test.go

@@ -180,6 +180,46 @@ func TestUpdateUserToRestrictedDomain(t *testing.T) {
 	})
 }
 
+func TestUpdateUser(t *testing.T) {
+	th := Setup(t)
+	defer th.TearDown()
+
+	user := th.CreateUser()
+	group := th.CreateGroup()
+
+	t.Run("fails if the username matches a group name", func(t *testing.T) {
+		user.Username = *group.Name
+		u, err := th.App.UpdateUser(user, false)
+		require.NotNil(t, err)
+		require.Equal(t, "app.user.group_name_conflict", err.Id)
+		require.Nil(t, u)
+	})
+}
+
+func TestCreateUser(t *testing.T) {
+	th := Setup(t)
+	defer th.TearDown()
+
+	group := th.CreateGroup()
+
+	id := model.NewId()
+	user := &model.User{
+		Email:         "success+" + id + "@simulator.amazonses.com",
+		Username:      *group.Name,
+		Nickname:      "nn_" + id,
+		Password:      "Password1",
+		EmailVerified: true,
+	}
+
+	t.Run("fails if the username matches a group name", func(t *testing.T) {
+		user.Username = *group.Name
+		u, err := th.App.CreateUser(th.Context, user)
+		require.NotNil(t, err)
+		require.Equal(t, "app.user.group_name_conflict", err.Id)
+		require.Nil(t, u)
+	})
+}
+
 func TestUpdateUserActive(t *testing.T) {
 	th := Setup(t)
 	defer th.TearDown()