public abstract HashMap<Integer, T_Sess> loadAllRawSessionsOf(OmemoDevice userDevice, BareJid contact) throws IOException;
@Test
    public void loadAllRawSessionsReturnsEmptyMapTest() throws IOException {
        HashMap<Integer, T_Sess> sessions = store.loadAllRawSessionsOf(alice, bob.getJid());
        assertNotNull(sessions);
        assertEquals(0, sessions.size());
    }