Setup E2E testcases for GitHub plugin

[Yash-Chakerverti]

I'm hesitant to merge a PR without proper tests. It creates technical debt and reduces the quality of the code.

I would love to see if we can take more time to write good tests for plugins. Especially Go E2E are quite powerful, as they are written in the same language as most of the plugin code and can still cover the perspective of the end user. See mattermost/mattermost-plugin-msteams#458 as an example.

cc @wiggin77

Originally posted by @hanzei in #886 (comment)