Improve solvation testing

[ianmkenney]

While the tests currently do what we want, it would be a good idea to check more than just the fact that the solvation process has no errors.

What should be checked:

• [ ] Paths to solvated structures make sense
• [ ] The correct solvent is actually used
• [ ] Multiple solvated structures can coexist in the same 'calculation' filesystem

[orbeckst]

Was this part of the PR #39 ?

[ianmkenney]

This has not been included yet. I wanted to expand these tests after covering a few of the other steps in our workflow.