pointFromX: add failing tests

This pull request adds failing (and passing) tests for `pointFromX`, which given the odd/even case parameter and `X` coordinate, should recover the `X, Y` coordinates accordingly.

I couldn't see anything wrong in the implementation, as it appears to be implemented verbatim as seen in the SEC paper.  However the results speak for themselves.

A reference (python) implementation using the `p/4` technique is shown here: https://gist.github.com/dcousens/e5ede344b05eec6d879c