ADD more test cases to improve test coverage

Improving the test coverage will help us move more smoothly when doing code migrations and save some time from debugging. Specifically, we may want to test features that require external services like GitHub by mocking responses.

Setting up a workable GitHub organization with a bot user in the local development environment is troublesome and we can't use the token for automated tests. So coming up with a reasonable white-box testing infrastructure for these features can be crucial for maintaining the functionality API server. It will prevent us from bringing broken function calls or backward-incompatible changes to the codebase. I think this will also be helpful for #281.
```
----------- coverage: platform linux, python 3.9.5-final-0 -----------
Name Stmts Miss Cover
---------------------------------------------------------------------------------------
/home/pig208/Anubis/api/anubis/__init__.py 0 0 100%
/home/pig208/Anubis/api/anubis/app.py 33 10 70%
/home/pig208/Anubis/api/anubis/config.py 32 4 88%
/home/pig208/Anubis/api/anubis/k8s/__init__.py 0 0 100%
/home/pig208/Anubis/api/anubis/k8s/pipeline.py 29 20 31%
/home/pig208/Anubis/api/anubis/k8s/theia.py 189 167 12%
/home/pig208/Anubis/api/anubis/lms/__init__.py 0 0 100%
/home/pig208/Anubis/api/anubis/lms/assignments.py 112 85 24%
/home/pig208/Anubis/api/anubis/lms/autograde.py 48 34 29%
/home/pig208/Anubis/api/anubis/lms/courses.py 161 122 24%
/home/pig208/Anubis/api/anubis/lms/lectures.py 13 6 54%
/home/pig208/Anubis/api/anubis/lms/questions.py 137 115 16%
/home/pig208/Anubis/api/anubis/lms/repos.py 8 2 75%
/home/pig208/Anubis/api/anubis/lms/students.py 17 9 47%
/home/pig208/Anubis/api/anubis/lms/submissions.py 128 110 14%
/home/pig208/Anubis/api/anubis/lms/theia.py 38 18 53%
/home/pig208/Anubis/api/anubis/lms/webhook.py 35 29 17%
/home/pig208/Anubis/api/anubis/models/__init__.py 355 59 83%
/home/pig208/Anubis/api/anubis/rpc/__init__.py 0 0 100%
/home/pig208/Anubis/api/anubis/rpc/batch.py 7 3 57%
/home/pig208/Anubis/api/anubis/rpc/lms.py 17 12 29%
/home/pig208/Anubis/api/anubis/rpc/pipeline.py 31 23 26%
/home/pig208/Anubis/api/anubis/rpc/seed.py 53 47 11%
/home/pig208/Anubis/api/anubis/rpc/theia.py 62 48 23%
/home/pig208/Anubis/api/anubis/rpc/visualizations.py 18 8 56%
/home/pig208/Anubis/api/anubis/utils/__init__.py 0 0 100%
/home/pig208/Anubis/api/anubis/utils/auth/__init__.py 0 0 100%
/home/pig208/Anubis/api/anubis/utils/auth/http.py 49 26 47%
/home/pig208/Anubis/api/anubis/utils/auth/oauth.py 5 0 100%
/home/pig208/Anubis/api/anubis/utils/auth/token.py 17 9 47%
/home/pig208/Anubis/api/anubis/utils/auth/user.py 34 22 35%
/home/pig208/Anubis/api/anubis/utils/cache.py 5 1 80%
/home/pig208/Anubis/api/anubis/utils/config.py 19 12 37%
/home/pig208/Anubis/api/anubis/utils/data.py 118 83 30%
/home/pig208/Anubis/api/anubis/utils/exceptions.py 27 11 59%
/home/pig208/Anubis/api/anubis/utils/github/__init__.py 0 0 100%
/home/pig208/Anubis/api/anubis/utils/github/api.py 49 41 16%
/home/pig208/Anubis/api/anubis/utils/github/fix.py 75 75 0%
/home/pig208/Anubis/api/anubis/utils/github/parse.py 12 8 33%
/home/pig208/Anubis/api/anubis/utils/github/repos.py 92 79 14%
/home/pig208/Anubis/api/anubis/utils/http/__init__.py 44 29 34%
/home/pig208/Anubis/api/anubis/utils/http/decorators.py 71 47 34%
/home/pig208/Anubis/api/anubis/utils/http/files.py 32 24 25%
/home/pig208/Anubis/api/anubis/utils/logging.py 7 0 100%
/home/pig208/Anubis/api/anubis/utils/migrate.py 6 2 67%
/home/pig208/Anubis/api/anubis/utils/pipeline/__init__.py 0 0 100%
/home/pig208/Anubis/api/anubis/utils/pipeline/decorators.py 22 22 0%
/home/pig208/Anubis/api/anubis/utils/rpc.py 39 22 44%
/home/pig208/Anubis/api/anubis/utils/testing/__init__.py 0 0 100%
/home/pig208/Anubis/api/anubis/utils/testing/autograde_timings.py 54 54 0%
/home/pig208/Anubis/api/anubis/utils/testing/seed.py 90 68 24%
/home/pig208/Anubis/api/anubis/utils/visuals/__init__.py 0 0 100%
/home/pig208/Anubis/api/anubis/utils/visuals/assignments.py 84 68 19%
/home/pig208/Anubis/api/anubis/utils/visuals/queries.py 4 0 100%
/home/pig208/Anubis/api/anubis/utils/visuals/usage.py 61 47 23%
/home/pig208/Anubis/api/anubis/views/__init__.py 0 0 100%
/home/pig208/Anubis/api/anubis/views/admin/__init__.py 19 0 100%
/home/pig208/Anubis/api/anubis/views/admin/assignments.py 110 54 51%
/home/pig208/Anubis/api/anubis/views/admin/auth.py 19 8 58%
/home/pig208/Anubis/api/anubis/views/admin/autograde.py 70 38 46%
/home/pig208/Anubis/api/anubis/views/admin/config.py 29 15 48%
/home/pig208/Anubis/api/anubis/views/admin/courses.py 138 78 43%
/home/pig208/Anubis/api/anubis/views/admin/dangling.py 28 9 68%
/home/pig208/Anubis/api/anubis/views/admin/ide.py 80 40 50%
/home/pig208/Anubis/api/anubis/views/admin/late_exceptions.py 58 35 40%
/home/pig208/Anubis/api/anubis/views/admin/lectures.py 74 46 38%
/home/pig208/Anubis/api/anubis/views/admin/questions.py 108 61 44%
/home/pig208/Anubis/api/anubis/views/admin/regrade.py 82 48 41%
/home/pig208/Anubis/api/anubis/views/admin/seed.py 15 3 80%
/home/pig208/Anubis/api/anubis/views/admin/static.py 33 11 67%
/home/pig208/Anubis/api/anubis/views/admin/students.py 77 40 48%
/home/pig208/Anubis/api/anubis/views/admin/visuals.py 35 14 60%
/home/pig208/Anubis/api/anubis/views/pipeline/__init__.py 5 5 0%
/home/pig208/Anubis/api/anubis/views/pipeline/pipeline.py 76 76 0%
/home/pig208/Anubis/api/anubis/views/public/__init__.py 19 0 100%
/home/pig208/Anubis/api/anubis/views/public/assignments.py 20 4 80%
/home/pig208/Anubis/api/anubis/views/public/auth.py 110 70 36%
/home/pig208/Anubis/api/anubis/views/public/courses.py 44 19 57%
/home/pig208/Anubis/api/anubis/views/public/ide.py 91 53 42%
/home/pig208/Anubis/api/anubis/views/public/lectures.py 14 3 79%
/home/pig208/Anubis/api/anubis/views/public/memes.py 7 2 71%
/home/pig208/Anubis/api/anubis/views/public/profile.py 26 13 50%
/home/pig208/Anubis/api/anubis/views/public/questions.py 40 18 55%
/home/pig208/Anubis/api/anubis/views/public/repos.py 60 29 52%
/home/pig208/Anubis/api/anubis/views/public/static.py 18 7 61%
/home/pig208/Anubis/api/anubis/views/public/submissions.py 51 29 43%
/home/pig208/Anubis/api/anubis/views/public/visuals.py 18 7 61%
/home/pig208/Anubis/api/anubis/views/public/webhook.py 71 51 28%
---------------------------------------------------------------------------------------
TOTAL 4184 2567 39%
```
(test coverage for `api/anubis`)