MNT: Fix failing xarray with units test

It's only by accident that this test even passed in the olden days
(because we were swallowing an AttributeError. This seems like the right
test for the actual expected functionality.

tests/test_xarray.py

@@ -114,8 +114,9 @@ def test_convert_units(test_var):
     assert units(test_var.attrs['units']) == units('degC')
 
     # Make sure we now get an array back with properly converted values
-    assert test_var.metpy.unit_array.units == units.degC
-    assert_almost_equal(test_var[0, 0, 0, 0], 18.44 * units.degC, 2)
+    quant = test_var.metpy.unit_array
+    assert quant.units == units.degC
+    assert_almost_equal(quant[0, 0, 0, 0], 18.44 * units.degC, 2)
 
 
 def test_convert_coordinate_units(test_ds_generic):