Bug in test for stress

[AkihideHayashi]

In test_torch_dftd3_calculator.py/_assert_energy_force_stress_equal, there is a code below.

    if np.all(atoms.pbc == np.array([True, True, True])):
        s1 = atoms.get_stress()
        s2 = atoms.get_stress()
        assert np.allclose(s1, s2, atol=1e-5, rtol=1e-5)

This code cannot compare the results of stresses of calc1 and calc2.
Both s1 and s2 are the stress of calc2.