Update tests: nroots does not take the error arg

Instead it raises an exception if the root is not converged.
Put back the original ordering

This is required by tests like the following:

    f = 7*z**4 - 19*z**3 + 20*z**2 + 17*z + 20

    assert intervals(f) == []

    real_part, complex_part = intervals(f, all=True, sqf=True)

    assert real_part == []
    assert all(re(a) < re(r) < re(b) and im(
        a) < im(r) < im(b) for (a, b), r in zip(complex_part, nroots(f)))

For now, we do not want to modify the symbolic algorithms in intervals(), so we
returned to the original ordering. We documented this in the relevant parts of
the code and tests.
Make the coefficients working for other fields

This just replicates the logic that was used there before for such fields.