Don't use another main test file as auxiliary

In this case, the exact extern crate isn't very important.

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.

Author: jieyouxu

tests/ui/rust-2018/removing-extern-crate-malformed-cfg.fixed

@@ -1,5 +1,5 @@
 //@ edition:2018
-//@ aux-build:../removing-extern-crate.rs
+//@ aux-build: remove-extern-crate.rs
 //@ run-rustfix
 
 #![warn(rust_2018_idioms)]

tests/ui/rust-2018/removing-extern-crate-malformed-cfg.rs

@@ -1,16 +1,16 @@
 //@ edition:2018
-//@ aux-build:../removing-extern-crate.rs
+//@ aux-build: remove-extern-crate.rs
 //@ run-rustfix
 
 #![warn(rust_2018_idioms)]
 
 #[cfg_attr(test, "macro_use")] //~ ERROR expected
-extern crate removing_extern_crate as foo; //~ WARNING unused extern crate
+extern crate remove_extern_crate as foo; //~ WARNING unused extern crate
 extern crate core; //~ WARNING unused extern crate
 
 mod another {
     #[cfg_attr(test)] //~ ERROR expected
-    extern crate removing_extern_crate as foo; //~ WARNING unused extern crate
+    extern crate remove_extern_crate as foo; //~ WARNING unused extern crate
     extern crate core; //~ WARNING unused extern crate
 }
 

tests/ui/rust-2018/removing-extern-crate-malformed-cfg.stderr

@@ -19,8 +19,8 @@ LL |     #[cfg_attr(test)]
 warning: unused extern crate
   --> $DIR/removing-extern-crate-malformed-cfg.rs:8:1
    |
-LL | extern crate removing_extern_crate as foo;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
+LL | extern crate remove_extern_crate as foo;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
    |
 note: the lint level is defined here
   --> $DIR/removing-extern-crate-malformed-cfg.rs:5:9
@@ -31,7 +31,7 @@ LL | #![warn(rust_2018_idioms)]
 help: remove the unused `extern crate`
    |
 LL - #[cfg_attr(test, "macro_use")]
-LL - extern crate removing_extern_crate as foo;
+LL - extern crate remove_extern_crate as foo;
 LL +
    |
 
@@ -50,13 +50,13 @@ LL +
 warning: unused extern crate
   --> $DIR/removing-extern-crate-malformed-cfg.rs:13:5
    |
-LL |     extern crate removing_extern_crate as foo;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
+LL |     extern crate remove_extern_crate as foo;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unused
    |
 help: remove the unused `extern crate`
    |
 LL -     #[cfg_attr(test)]
-LL -     extern crate removing_extern_crate as foo;
+LL -     extern crate remove_extern_crate as foo;
 LL +
    |