Our tests don't consistently report what file is missing

For example, I'm importing into google3 and getting things like

```shell
---- color::traversal_tests::clipbox_default stdout ----
thread 'color::traversal_tests::clipbox_default' panicked at third_party/rust/skrifa/v0_15/src/color/traversal_tests/mod.rs:306:46:
Unable to open expectations file.: Os { code: 2, kind: NotFound, message: "No such file or directory" }
```

It's sufficient, but saying which file would be more helpful.

Start in third_party/rust/skrifa/v0_15/src/color/traversal_tests/mod.rs around line 306 and reproduce color::traversal_tests::clipbox_default. Trace the expectations-file open error; done means the failure reports which file is missing, while the test still passes when that file exists.

Written by the indexing model from the issue text.