Use serialized inputs to test the hybrid topology factory

[jthorton]

Rather than doing all this, and being at risk of dealing with flakyness with variabilities in system building, why not just serialize the two end state systems? This is the approach openmmtools does for factory testing, so do we internally for some of the absolute factory things, it would make life a ton easier.

Originally posted by @IAlibay in #1695 (comment)

[jthorton]

I had a quick go at this with the CMAP test system in #1695 and found that the .bz2 systems were around 1MB combined (as we need two systems and pdbs per test), so with many variants on this type of test this could significantly increase the size of the repo unless they are stored remotely.