Add test cases to dihedral conversions in conversion.py

[bc118]

Since we are removing MoSDeF-GOMC from mBuild, it means many of the checks in the GOMC/Charmm writers for these dihedrals will be removed. They may be fully or partially covered by other writers; however, we should make explicit tests for them in a separate test file.

This is the file that includes the dihedrals and needs a test file (checked):

• mbuild/utils/conversion.py