message: Fix windows failure on collision_dylib.
diff: @@ -39,7 +39,9 @@ fn collision_dylib() {
  .file("b/src/lib.rs", "")
  .build();
  - p.cargo("build")
  + // j=1 is required because on windows you'll get an error because
  + // two processes will be writing to the file at the same time.
  + p.cargo("build -j=1")
  .with_stderr_contains(&format!("\
  [WARNING] output filename collision.
  The lib target `a` in package `b v1.0.0 ([..]/foo/b)` has the same output filename as the lib target `a` in package `a v1.0.0 ([..]/foo/a)`.
  