feat: worker convenience methods (#2677)

This patch:
- adds `worker.evaluate` and `worker.evaluateHandle` methods as a shortcut to their execution context equivalents.
- makes the error messages a bit nicer when interacting with a closed worker (as opposed to a closed page).
- moves the worker tests into their own spec file.

lib/Worker.js

@@ -53,6 +53,24 @@ class Worker extends EventEmitter {
   async executionContext() {
     return this._executionContextPromise;
   }
+
+  /**
+   * @param {function()|string} pageFunction
+   * @param {!Array<*>} args
+   * @return {!Promise<*>}
+   */
+  async evaluate(pageFunction, ...args) {
+    return (await this._executionContextPromise).evaluate(pageFunction, ...args);
+  }
+
+  /**
+   * @param {function()|string} pageFunction
+   * @param {!Array<*>} args
+   * @return {!Promise<!JSHandle>}
+   */
+  async evaluateHandle(pageFunction, ...args) {
+    return (await this._executionContextPromise).evaluateHandle(pageFunction, ...args);
+  }
 }
 
 module.exports = Worker;