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

[aryan26roy]

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.