Fix test_nonnumeric_magnitudes

[labrys]

The test for non-numeric magnitudes was using assertRaises incorrectly. While the context-manager form should be preferred, with the given style it should be passed the function to test, not the result of the function.

# Wrong
self.assertRaises(RuntimeError, self.compareQuantity_compact(x,x))

# Better
self.assertRaises(RuntimeError, self.compareQuantity_compact, x, x)

# Best
with self.assertRaises(RuntimeError):
    self.compareQuantity_compact(x,x)

While fixing this I found that RuntimeError was not being raised, instead a RuntimeWarning was being issued. Since unittest.TestCase.assertWarns was not added until Python 3.2 (see docs), I'm testing for the warning as recommended in the Python 2.7 docs.

Fixes #659

[hgrecco]

I think this PR is no longer necessary due to the recent merge of #910