TST: Make calc factory for lammpslib

Currently, these tests generally fail unless you've made sure that lammps can find the necessary potential file. This can be done by placing the file in the current directory or elsewhere in the python search path, or by setting the LAMMPS_POTENTIALS environment variable (see https://lammps.sandia.gov/doc/pair_python.html). EDIT:Because the new ase-dockers lammps image installs the bundled potential parameter files, we can place a symlink to the relevant file in the current directory.

Rather than bother with symlinks or environment variables, we now make the necessary potential files available to the tests via ase-datafiles, which is installed in the docker full-monty image. This is accomplished by creating a calculator factory pytest fixture for the lammpslib calculator. Also changed in this MR are the lammpslib tests themselves, which have been adapted to conform to pytest.