ENH: Always have PBC on resulting atoms object from VASP, and add test

As requested my @mjwaters, the VASP calculator should return an atoms object which always has pbc=True. Added a test for this.

Also adjusted a test, so we didn't expect a specific energy, as that's VASP version and setup specific.

A possible discussion point, is whether this should raise a warning, but since we already "freely" mutate the atoms objects during geometry optimizations, I felt it was ok to just mutate this.