message: Fix collision doc tests randomly failing.
diff: @@ -146,7 +146,7 @@ fn collision_doc() {
  .file("foo2/src/lib.rs", "")
  .build();
  - p.cargo("doc")
  + p.cargo("doc -j=1")
  .with_stderr_contains(
  "\
  [WARNING] output filename collision.
  @@ -407,7 +407,7 @@ fn collision_doc_sources() {
  .file("bar/src/lib.rs", "")
  .build();
  - p.cargo("doc")
  + p.cargo("doc -j=1")
  .with_stderr_unordered(
  "\
  [UPDATING] [..]
  @@ -524,7 +524,7 @@ fn collision_with_root() {
  .file("foo-macro/src/lib.rs", "")
  .build();
  - p.cargo("doc")
  + p.cargo("doc -j=1")
  .with_stderr_unordered("\
  [UPDATING] [..]
  [DOWNLOADING] crates ...
  