message: Fix tests on beta
  Had a few usages of `-Z` leak out of the nightly channel.
diff: @@ -2358,6 +2358,8 @@ fn custom_target_dir() {
  #[test]
  fn rustc_no_trans() {
  + if !is_nightly() { return }
  +
  let p = project("foo")
  .file("Cargo.toml", r#"
  [package]
  @@ -2689,16 +2691,13 @@ r#"[ERROR] Could not match 'xml' with any of the allowed variants: ["Human", "Js
  #[test]
  fn message_format_json_forward_stderr() {
  - if is_nightly() { return }
  -
  let p = project("foo")
  .file("Cargo.toml", &basic_bin_manifest("foo"))
  .file("src/main.rs", "fn main() { let unused = 0; }");
  assert_that(p.cargo_process("rustc").arg("--bin").arg("foo")
  - .arg("--message-format").arg("JSON").arg("--").arg("-Zno-trans"),
  + .arg("--message-format").arg("JSON"),
  execs().with_status(0)
  - .with_stderr_contains("[WARNING] the option `Z` is unstable [..]")
  .with_json(r#"
  {
  "reason":"compiler-message",
  @@ -2729,7 +2728,7 @@ fn message_format_json_forward_stderr() {
  "test":false
  },
  "features":[],
  - "filenames":[],
  + "filenames":["[..]"],
  "fresh": false
  }
  "#));
  