Write preprocessed content to file before testing with rustdoc

[carols10cents]

Rather than writing the unpreprocessed content to the file for rustdoc to test with.

Fixes #855.

I was trying to figure out whether the current behavior was intentional or not, and if I might be breaking anything with this change, but I think the fix I have here is what was intended-- my first hint was this comment that described what I had hoped was happening but that wasn't what was actually happening.

As of this commit, it seems like the behavior I was hoping for was happening and was intended ("now mdbook test does full link expansion to temp file prior to running" in the commit message), but this line got lost in refactoring and the preprocessors were being run but this line read the unprocessed file from disk again and gave that content to rustdoc :(

If this fails CI I will fix it tomorrow ❤️

[carols10cents]

Wooo ok it passed! I was second guessing myself about the tests after submitting this, so in case anyone has the same thoughts I did: it would appear that the mdbook_can_correctly_test_a_passing_book would pass even if nested-test.rs wasn't included, but it fails without the included code change because rustdoc can't parse {{#include... as Rust code.

The mdbook_detects_book_with_failing_tests would be better if it tested that there were 2 failures rather than just that the tests failed, but I'm not sure how to do that.

[ehuss]

Thanks! Sorry I lost track of this.