Appveyor ctypes test failures

[njwilson23]

Tests that use ctypes to test functions implemented in C are failing on Appveyor:

python tests\runtests.py
..........EEE...............................................................................................................................................................E.................................sssss.ss.ss............................................................................................................................................................
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ERROR: test_bbox_intersection_area (rtree_tests.CRTreeTests)
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Traceback (most recent call last):
  File "C:\projects\karta\tests\rtree_tests.py", line 54, in test_bbox_intersection_area
    self.rt.bbox_intersection_area.restype = c_float
  File "C:\Miniconda3\envs\test-environment\lib\ctypes\__init__.py", line 360, in __getattr__
    func = self.__getitem__(name)
  File "C:\Miniconda3\envs\test-environment\lib\ctypes\__init__.py", line 365, in __getitem__
    func = self._FuncPtr((name_or_ordinal, self))
AttributeError: function 'bbox_intersection_area' not found
======================================================================
ERROR: test_is_overlapping (rtree_tests.CRTreeTests)
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Traceback (most recent call last):
  File "C:\projects\karta\tests\rtree_tests.py", line 39, in test_is_overlapping
    self.rt.is_overlapping.restype = c_int
  File "C:\Miniconda3\envs\test-environment\lib\ctypes\__init__.py", line 360, in __getattr__
    func = self.__getitem__(name)
  File "C:\Miniconda3\envs\test-environment\lib\ctypes\__init__.py", line 365, in __getitem__
    func = self._FuncPtr((name_or_ordinal, self))
AttributeError: function 'is_overlapping' not found
======================================================================
ERROR: test_is_within (rtree_tests.CRTreeTests)
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Traceback (most recent call last):
  File "C:\projects\karta\tests\rtree_tests.py", line 24, in test_is_within
    self.rt.is_within.restype = c_int
  File "C:\Miniconda3\envs\test-environment\lib\ctypes\__init__.py", line 360, in __getattr__
    func = self.__getitem__(name)
  File "C:\Miniconda3\envs\test-environment\lib\ctypes\__init__.py", line 365, in __getitem__
    func = self._FuncPtr((name_or_ordinal, self))
AttributeError: function 'is_within' not found
======================================================================
ERROR: test_iswithin (quadtree_tests.TestCQuadTree)
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Traceback (most recent call last):
  File "C:\projects\karta\tests\quadtree_tests.py", line 25, in test_iswithin
    self.qt.iswithin.restype = c_bool
  File "C:\Miniconda3\envs\test-environment\lib\ctypes\__init__.py", line 360, in __getattr__
    func = self.__getitem__(name)
  File "C:\Miniconda3\envs\test-environment\lib\ctypes\__init__.py", line 365, in __getitem__
    func = self._FuncPtr((name_or_ordinal, self))
AttributeError: function 'iswithin' not found
----------------------------------------------------------------------
Ran 373 tests in 4.453s
FAILED (errors=4, skipped=9)

The Python tests using the C functions as an extension module are passing, so this is probably a problem with how the tests are set up, rather than with the build.

Since I don't use a Windows machine, this is a little tricky to work on.