Await file operations when copying test data

Both `copyNoWorkspaceData` and `copyCliIntegrationData` return
promises. Since file copy-ing is quite fast at the moment, this
hasn't been a problem, but it might become a problem in the future
if we start copying larger files.

Let's wait for the operations to finish.

extensions/ql-vscode/gulpfile.ts/tests.ts

@@ -1,9 +1,10 @@
 import * as gulp from 'gulp';
 
 export function copyTestData() {
-  copyNoWorkspaceData();
-  copyCliIntegrationData();
-  return Promise.resolve();
+  return Promise.all([
+    copyNoWorkspaceData(),
+    copyCliIntegrationData()
+  ]);
 }
 
 export function watchTestData() {