Add unit tests for various 'web-sys' HTML element bindings

That list includes:
 * HtmlOptionsCollection
 * HtmlOutputElement
 * HtmlParagraphElement
 * HtmlParamElement
 * HtmlPreElement
 * HtmlProgressElement
 * HtmlQuoteElement
 * HtmlSlotElement

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

@@ -0,0 +1,21 @@
+use wasm_bindgen_test::*;
+use wasm_bindgen::prelude::*;
+use web_sys::HtmlProgressElement;
+
+#[wasm_bindgen(module = "./tests/wasm/element.js")]
+extern {
+    fn new_progress() -> HtmlProgressElement;
+}
+
+#[wasm_bindgen_test]
+fn test_progress_element() {
+    let progress = new_progress();
+    progress.set_max(150.5);
+    assert_eq!(progress.max(), 150.5, "Maximum progress value should be 150.5.");
+
+    progress.set_value(22.);
+    assert_eq!(progress.value(), 22., "Progress value should be 22 units.");
+    assert_eq!(progress.position(), (22. / 150.5), "Progress position should be 22 divided by the max possible value.");
+
+    assert!(progress.labels().length() == 0, "Our simple progress bar shouldn't be associated with any labels.");
+}