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.

Test vectors from: https://github.com/cryptocoinjs/ecurve/blob/master/test/fixtures/point.json