compiletest: the `aux-crate` directive can't deal with more complex paths it seems

I've chatted with @jieyouxu and we've come to the conclusion that it's worthwhile to track compiletest issues even though we plan on fully migrating to ui_test eventually since that might still take a while.

While //@ aux-build:../helper.rs[^1] works as one would expect (compiletest finds ./auxiliary/../helper.rs aka ./helper.rs), //@ aux-crate:helper=../helper.rs does not. It leads to the following rustc error:

error: extern location for helper does not exist: <REDACTED>/test/ui/dummy/auxiliary/lib../helper.so
  --> <REDACTED>/tests/ui/dummy.rs:4:1
   |
LL | extern crate helper;
   | ^^^^^^^^^^^^^^^^^^^^

error: aborting due to 1 previous error

Reproducer:

tests/ui/helper.rs: empty.

tests/ui/dummy.rs:

//@ aux-crate:helper=../helper.rs
//@ check-pass
extern crate helper;
fn main() {}

[^1]: Indeed, this pattern is used inside tests/ui/!