Migrate webidl tests to wasm_bindgen_test (#590)

This commit moves the `webidl/tests` folder to a new `crates/webidl-tests` crate
(to have a test-only build script) and ports them to the `#[wasm_bindgen_test]`
attribute, which should hopefully make testing much speedier for execution!

crates/webidl-tests/enums.js

@@ -0,0 +1,21 @@
+global.Shape = class Shape {
+  constructor(kind) {
+    this.kind = kind;
+  }
+
+  static triangle() {
+    return new Shape('triangle');
+  }
+
+  isSquare() {
+    return this.kind === 'square';
+  }
+
+  isCircle() {
+    return this.kind === 'circle';
+  }
+
+  getShape() {
+    return this.kind;
+  }
+};