Don't use another main test file as auxiliary
In this case, the exact extern crate isn't very important, it just needs to not be another main test file.
This is part of the changes needed to address the spurious failures from a main test `../removing-extern-crate.rs` being both an auxiliary and a main test file, causing fs races due to multiple `rustc` processes in multiple test threads trying to build the main test file both as a main test and also as an auxiliary at around the same time.
Part 1 of rust-lang/rust#144237.
r? ``@RalfJung`` (or compiler)
55a477efb2