[MM-28195] storetest/user_store: fix flaky test (#15543)

* storetest/user_store: fix flaky test

* reflect review comments

store/storetest/user_store.go

@@ -2321,7 +2321,7 @@ func testUserStoreGetNewUsersForTeam(t *testing.T, ss store.Store) {
 
 	u1, err := ss.User().Save(&model.User{
 		Email:    MakeEmail(),
-		Username: "u1" + model.NewId(),
+		Username: "Yuka",
 	})
 	require.Nil(t, err)
 	defer func() { require.Nil(t, ss.User().PermanentDelete(u1.Id)) }()
@@ -2330,7 +2330,7 @@ func testUserStoreGetNewUsersForTeam(t *testing.T, ss store.Store) {
 
 	u2, err := ss.User().Save(&model.User{
 		Email:    MakeEmail(),
-		Username: "u2" + model.NewId(),
+		Username: "Leia",
 	})
 	require.Nil(t, err)
 	defer func() { require.Nil(t, ss.User().PermanentDelete(u2.Id)) }()
@@ -2339,7 +2339,7 @@ func testUserStoreGetNewUsersForTeam(t *testing.T, ss store.Store) {
 
 	u3, err := ss.User().Save(&model.User{
 		Email:    MakeEmail(),
-		Username: "u3" + model.NewId(),
+		Username: "Ali",
 	})
 	require.Nil(t, err)
 	defer func() { require.Nil(t, ss.User().PermanentDelete(u3.Id)) }()