Move reusable data and utilities into testsystems

[mcwitt]

This was partially addressed in #696, but there is potentially much more we could do.

From #696 (comment):

• DHFR, maybe others from test_benchmark.py ? (seems useful)
• BenzenePhenolSparseTest in test_topology.py ? (unclear)
• make_gaussian_testsystem in test_reweighting.py ? (maybe useful for testing protocol optimization)
• some of the paired smiles in test_rabfe_topology.py ? (maybe useful for stress-testing single-topology methods)
• the log weights functions etc. in test_mtm.py::test_optimized_MTM (the batching and other transformations may be useful to extract to testsystems (or examples or md.enhanced))
• get_cdk8_coords etc in test_bonded.py ? (unclear)
• there are also many molecules defined inline in test_dummy.py, but I don't immediately know which exercise corner cases that could be relevant elsewhere (e.g. if this contains molecules that exercise the "collinear angles" corner case, maybe these are useful also in testing the HarmonicAngle implementation?)