Add tests for Canvas Integration plugin

[arslanashraf7]

Background:

We recently worked on creating a new plugin for our canvas integration with edX. The plugin can be seen in this PR #8.

What needs to be done?

For now, we didn't add unit tests since they might require a reasonable amount of time to get done since we might see some hurdles creating the tests that will run within the edx-platform and they were not already a part of the canvas_integration Django app when we created that within the edx-platform (Reading the description in #8 might help).

Possible Approach:

Maybe we can use the same approach as rapid-response-xblock to write tests and set up the environment for them. You can see this & this reference from Rapid Response to get an idea of how it's done there.