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

Automatically provide a resource reader for tests

Fix #20710

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- [x] These changes fix #20710 (github issue number if applicable).

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components/script/Cargo.toml

@@ -103,3 +103,6 @@ webvr_traits = {path = "../webvr_traits"}
 
 [target.'cfg(not(target_os = "ios"))'.dependencies]
 mozangle = "0.1"
+
+[dev-dependencies]
+embedder_traits = { path = "../embedder_traits", features = ["tests"] }