ci: fix wasm-bindgen warning on nightly

See mpalmer#78
See rustwasm/wasm-bindgen#4283

Author: bcheidemann

Cargo.toml

@@ -47,3 +47,6 @@ wasm-bindgen-test = "0.3.13"
 [profile.release]
 # Tell `rustc` to optimize for small code size.
 opt-level = "s"
+
+[lints.rust]
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(wasm_bindgen_unstable_test_coverage)'] }