Add assert methods for drawing and compiled paths

[itziakos]

This PR adds a couple of assert methods in enable test assistant to help test path creation and drawing methods.

[coveralls]

Coverage remained the same when pulling dc34da1 on enh-path-assertion-methods into f618921 on master.

[mdickinson]

I suggest naming this create_mock_gc, for consistency with create_mock_window.

[mdickinson]

(Or perhaps even create_mock_graphics_context, though gc seems sufficiently clear in this context.)

[itziakos]

I will go with create_mock_gc

[itziakos]

Done

[itziakos]

I need to add a few tests using the new assert methods.

[coveralls]

Coverage remained the same when pulling c0b8e2e on enh-path-assertion-methods into f618921 on master.

[coveralls]

Coverage decreased (-1.48%) when pulling 34b0e34 on enh-path-assertion-methods into f618921 on master.

[coveralls]

Coverage remained the same when pulling 272c4b9 on enh-path-assertion-methods into f618921 on master.

[itziakos]

@jwiggins and @mdickinson, I have updated the PR. Please have another look and let me know if I have missed anything.

note: I have factored out the graphic context assert methods into the kiva subpackage where I think they belong.

[coveralls]

Coverage remained the same when pulling 7b12d9a on enh-path-assertion-methods into f618921 on master.

[coveralls]

Coverage remained the same when pulling 7b12d9a on enh-path-assertion-methods into f618921 on master.

[coveralls]

Coverage increased (+0.03%) when pulling 4761ec7 on enh-path-assertion-methods into f618921 on master.

[coveralls]

Coverage remained the same when pulling 6252681 on enh-path-assertion-methods into f618921 on master.

[coveralls]

Coverage remained the same when pulling 6252681 on enh-path-assertion-methods into f618921 on master.

[jwiggins]

LGTM