Move auxiliary directories to live with the tests
This is a step for enabling testing of cross-crate incremental compilation. The idea is that instead of having a central auxiliary directory, when you have a `// aux-build:foo.rs` annotation in the test `run-pass/bar.rs`, it will look in (e.g.) `run-pass/aux/foo.rs`. In general, it looks for an `aux` directory in the same directory as the test. We also ignore the `aux` directories when enumerating the set of tests.
As part of this PR, also refactor `runtest.rs` to use methods on a context, which means we can stop passing around context everywhere.
r? @alexcrichton
62e2b2fb7a