To test behaviour that depends on the extern options of intermediate
crates, compiletest auxiliaries must have their own auxiliaries.
Auxiliary compilation previously did not trigger compilation of any
auxiliaries in the auxiliary's headers. In addition, those auxiliaries
would need to be in an `auxiliary/auxiliary` directory, which is
unnecessary and makes some crate graphs harder to write tests for,
such as when A depends on B and C, and B depends on C.
For a test `tests/ui/$path/root.rs`, with the following crate graph:
```
root
|-- grandparent
`-- parent
`-- grandparent
```
then the intermediate outputs from compiletest will be:
```
build/$target/test/ui/$path/
|-- auxiliary
| |-- libgrandparent.dylib
| |-- libparent.dylib
| |-- grandparent
| | |-- grandparent.err
| | `-- grandparent.out
| `-- parent
| |-- parent.err
| `-- parent.out
|-- libroot.rmeta
|-- root.err
`-- root.out
```
Signed-off-by: David Wood <david@davidtw.co>
