compiletest: Don't assume `aux-crate` becomes a `*.so` with `no-prefer-dynamic` Since it does not make sense to do so. If someone prefers no dynamic stuff, the last thing they want to look for is an .so file. Also add a regression test. Without the fix, the test fails with: error: test compilation failed although it shouldn't! --- stderr ------------------------------- error: extern location for no_prefer_dynamic_lib does not exist: .../auxiliary/libno_prefer_dynamic_lib.so --> .../no-prefer-dynamic-means-no-so.rs:9:5 | LL | no_prefer_dynamic_lib::return_42(); | ^^^^^^^^^^^^^^^^^^^^^ ### Needed by: - https://github.com/rust-lang/rust/pull/150591 because of https://github.com/rust-lang/rust/issues/151271. But IMHO this PR makes sense on its own. ### Must wait for: - [x] https://github.com/rust-lang/rust/pull/151695