Ultimate test of parsing

[dtolnay]

Write a test to:

1. Iterate through Rust files in a directory
2. Parse using syn >> print using syn >> parse using syntex
3. Parse the original using syntex
4. Assert that the syntex AST from 2 and 3 are identical

I think this would give us more confidence in the parser than any unit test suite we could write, and it is also far easier to implement than individually testing every parser element. Once we make more progress on #4 we could run this test against a large repo like the full rustc source code.

cc @gregkatz. I plan to work on this today or tomorrow. In the meantime you don't have to worry about unit testing parsers you write (unless it is useful to you in implementing them). Let's focus on flying through #4 without tests and leave testing to this one.

[gregkatz]

Sure thing. I'll skip the unit tests in the future.

[dtolnay]

Added in 4a51dc7. Any *.rs file in the tests/cases directory will be tested as outlined above.