testGetTile() checks against getPlane(). See #11315

Extend gatewaytest/pixels.py testGetTile() to check that we get the same pixel values for getTile() and the corresponding region of a plane, using getPlane().

This fails in FS but passes in dev_4_4: See ticket #11315.

To run test:
```
$ python gatewaytest/pixels.py PixelsTest.testGetTile
F
======================================================================
FAIL: testGetTile (__main__.PixelsTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "gatewaytest/pixels.py", line 101, in testGetTile
    self.assertEqual(str(testTile), str(croppedPlane), "Tile and croppedPlane not equal")
AssertionError: Tile and croppedPlane not equal
```