Add unit test to check ImportError is raised when openmm is not installed

[Atharva7K]

Is your feature request related to a problem?

#3511(after merging) will add daltons as a dependency to the OpenMMTopologyParser for which openmm library needs to be installed. But if it isn't installed, users should get an ImporError with the appropriate Error message. We'd like to have a unit test to test this behavior.

Describe the solution you'd like

Reformat the openmm parser tests so that the tests don't directly skip on import Add a unit test to test the raised error

Additional context

https://github.com/MDAnalysis/mdanalysis/pull/3511#discussion_r850173158

ping @IAlibay