message: Don't use .arg() in test.
diff: @@ -1488,7 +1488,7 @@ fn doc_edition() {
  .build();
  assert_that(
  - p.cargo("doc").arg("-v").masquerade_as_nightly_cargo(),
  + p.cargo("doc -v").masquerade_as_nightly_cargo(),
  execs()
  .with_status(0)
  .with_stderr_contains("[RUNNING] `rustdoc [..]-Zunstable-options --edition=2018[..]"),
  