test: use safer potential to test interaction
test: use safer potential to test interaction

Since Harmonic Potential has a indifferentiable point when combined with
dihedral interaction because of the periodicity in angle.
Use strictly periodic potential for testing.
Also, to avoid a numeric error, the initial positions of particles are
also changed.

ToruNiina/Mjolnir,ToruNiina/Mjolnir,ToruNiina/Mjolnir