message: Fix spurious error for test json_artifact_includes_test_flag
  There is a race condition for which job finishes first. On CI I am able
  to reproduce the error about 1% of the time.
  Recent failures:
diff: @@ -2992,32 +2992,9 @@ fn json_artifact_includes_test_flag() {
  ).file("src/lib.rs", "")
  .build();
  - p.cargo("test -v --message-format=json")
  + p.cargo("test --lib -v --message-format=json")
  .with_json(
  r#"
  - {
  - "reason":"compiler-artifact",
  - "profile": {
  - "debug_assertions": true,
  - "debuginfo": 2,
  - "opt_level": "0",
  - "overflow_checks": true,
  - "test": false
  - },
  - "executable": null,
  - "features": [],
  - "package_id":"foo 0.0.1 ([..])",
  - "target":{
  - "kind":["lib"],
  - "crate_types":["lib"],
  - "edition": "2015",
  - "name":"foo",
  - "src_path":"[..]lib.rs"
  - },
  - "filenames":["[..].rlib"],
  - "fresh": false
  - }
  -
  {
  "reason":"compiler-artifact",
  "profile": {
  