Move web-sys tests to the new test framework

Migrate most `web-sys` tests to the new `wasm_bindgen_test` framework with the
new headless browser capabilities!

crates/web-sys/tests/wasm/headers.rs

@@ -0,0 +1,31 @@
+use wasm_bindgen_test::*;
+use wasm_bindgen::prelude::*;
+use web_sys::Headers;
+
+#[wasm_bindgen(module = "./tests/wasm/headers.js")]
+extern {
+    fn new_headers() -> Headers;
+}
+
+#[wasm_bindgen_test]
+fn headers() {
+    let headers = new_headers();
+    assert_eq!(headers.get("foo").unwrap(), None);
+    assert_eq!(
+        headers.get("content-type").unwrap(),
+        Some("text/plain".to_string()),
+    );
+    assert_eq!(
+        headers.get("Content-Type").unwrap(),
+        Some("text/plain".to_string()),
+    );
+    assert!(headers.get("").is_err());
+    assert!(headers.set("", "").is_err());
+    assert!(headers.set("x", "").is_ok());
+    assert_eq!(headers.get("x").unwrap(), Some(String::new()));
+    assert!(headers.delete("x").is_ok());
+    assert_eq!(headers.get("x").unwrap(), None);
+    assert!(headers.append("a", "y").is_ok());
+    assert!(headers.append("a", "z").is_ok());
+    assert_eq!(headers.get("a").unwrap(), Some("y, z".to_string()));
+}