"celiagg" should be an optional dependency for kiva test run

Running the testsuite without `celiagg` should not cause an error - which it is at the moment.

```
======================================================================
ERROR: kiva.tests.test_celiagg_drawing (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: kiva.tests.test_celiagg_drawing
Traceback (most recent call last):
File "C:\Users\rporuri\.edm\envs\enable-test-release\lib\unittest\loader.py", line 428, in _find_test_path
module = self._get_module_from_name(name)
File "C:\Users\rporuri\.edm\envs\enable-test-release\lib\unittest\loader.py", line 369, in _get_module_from_name
__import__(name)
File "C:\Users\rporuri\.edm\envs\enable-test-release\lib\site-packages\kiva\tests\test_celiagg_drawing.py", line 12, in
from kiva.celiagg import GraphicsContext
File "C:\Users\rporuri\.edm\envs\enable-test-release\lib\site-packages\kiva\celiagg.py", line 17, in
import celiagg as agg
ModuleNotFoundError: No module named 'celiagg'
----------------------------------------------------------------------
```