tests: add vectorized roundtrip test to fixed/inertial planetary frames
tests: add vectorized roundtrip test to fixed/inertial planetary frames

This test ensures conversion from and to fixed frames is possible with
vectorized input and yields the same coordinates if coming back to the
original coordinate system.

Juanlu001/poliastro,Juanlu001/poliastro,poliastro/poliastro,Juanlu001/poliastro