Migrates to minitest and makes FedEx XML parsing failures more useful

The error messages from REXML leave a lot to be desired, especially since it says things failed at column x and row y, but doesn't give you the content.

This just improves the error message so we also have the response body from FedEx so that it's easier to debug.

Also wrapped `REXML::Document` creation in a function to keep things dry and such.

Please review @Soleone @jamesmacaulay /cc @jduff