Reframe the `examples/specular_example.py' as a validation test for specular point computation

[vinay0000]

This example calculates the specular point using the CYGNSS dataset, which includes the positions of CYGNSS and GPS satellites, for both spherical and ellipsoidal Earth models. The computed specular point is then compared with the specular point in the CYGNSS dataset (from CYGNSS mission).

Move the example into the tests or slowtests folder and implement a metric to evaluate the error between the specular point computed by OrbitPy and the specular point from the CYGNSS dataset. Ensure that the error is within an acceptable threshold value.
(Additionally, update the script’s documentation to accurately reflect the methodology. Mention the source of the CYGNSS dataset.)