End to end tests

[BurdetteLamar]

I think there's been some discussion about end-to-end testing. Here's a start.

There are two questions:

• Is the community interested in end-to-end testing (and, crucially, willing to review PRs)?
• Is the structure offered here workable? If not, what would be better?

[BurdetteLamar]

@peterzhu2118 and @st0012: I see that there are many failing checks. I'd like to wait for opinions about the test code before addressing the failures (If no one likes this PR, it needs to be closed, not fixed.)

[BurdetteLamar]

@st0012 and @peterzhu2118: The checks now all pass, so I think this is ready for review.

[st0012]

By end to end, I do mean running rdoc executable directly in tests 👍
But if we do that, we probably don't need most or all the helpers and stuff from XrefTestCase. Instead, we can create a new IntegrationTestCase that provides the run_rdoc method directly.

[BurdetteLamar]

To be replaced by #1430.