Remove network calls in test suite

This PR patches HTTP requests made by the test suite.  I found these calls by running the test suite with my wifi disabled and observing which tests failed.

I had to patch these calls to make the tests pass:
* Course input: Textbook TOC retrieval from S3
* SymbolicResponse: Snuggletex server calls

I also fixed some style issues and refactored how SymbolicResponse loads fixtures.

Suggested reviewers: @jzoldak @nedbat