[ENH] Test the pag_to_mag function with PAG containing "--o" edge.

**Is your feature request related to a problem? Please describe.**
Currently there is a code path in the function that requires a PAG with a --o edge.

**Describe the solution you'd like**
Generate a MAG-PAG pair using selection bias and add it as a test.

**Additional context**
This is leftover work from #93.
