Add regression test for resolving `--extern libc=test.rlib` Closes #26043 I could not find a test for this particular use case. The closest I got was [`tests/ui/imports/issue-37887.rs`](https://github.com/rust-lang/rust/blob/master/tests/ui/imports/issue-37887.rs), but that is a regression test for a different use case (see https://github.com/rust-lang/rust/issues/37887).
--extern libc=test.rlib