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]`.