[GH #926] Add a test for sort method of ResizableIntegerArray PMC
[GH #926] Extended sort() tests of ResizableIntegerArray PMC

The tests of the sort() method weren't testing that after resizing the
array, that the sort() method still works (the array is resizable after all
:-) ).  This commit adds these extra tests.
Merge pull request #946 from paultcochrane/ptc/resizable_int_array_sort_test

[GH #926] Add a test for sort method of ResizableIntegerArray PMC