Fix test error formatting to make values adjacent

I have stared blearily dozens of times at Skyfield test suite failures
that look like (at least when printed out by py.test):

    AssertionError: 1.4155283744703753
    does not equal
    1.415469450469653
    within the error bound ...

Why does it only now occur to me that these errors would be vastly
easier to read if formatted as follows?

    AssertionError:
    1.4155283744703753 does not equal
    1.415469450469653
    within the error bound ...

At least I thought of it eventually.