Auto merge of #20718 - paulrouget:res2, r=emilio

Automatically provide a resource reader for tests

Fix #20710

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [ ] `./mach build-geckolib` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #20710 (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/20718)
<!-- Reviewable:end -->

Cargo.lock

@@ -3089,7 +3089,6 @@ dependencies = [
  "app_units 0.6.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "byteorder 1.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "cssparser 0.23.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "embedder_traits 0.0.1",
  "euclid 0.17.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "html5ever 0.22.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "parking_lot 0.5.4 (registry+https://github.com/rust-lang/crates.io-index)",