libsyntax: Workaround pprust `for` issue
test: Add missing `#![feature(managed_boxes)]`

They use managed boxes, but are not correctly checked by feature gate
since they are inside macroes.
compiletest: Modernize typenames
compiletest: Test `--pretty expanded`

After testing `--pretty normal`, it tries to run `--pretty expanded` and
typecheck output.
Here we don't check convergence since it really diverges: for every
iteration, some extra lines (e.g.`extern crate std`) are inserted.

Some tests are `ignore-pretty`-ed.
Note that `--test` doesn't work with `--pretty expanded` since it prints
`#![!resolve_unexported]`.