ci(docs-infra): rename 'upload-server' to 'preview-server'

The server no longer has files uploaded to it. Instead it is more
accurate to refer to it as dealing with "previews" of PRs.

aio/aio-builds-setup/dockerbuild/scripts-js/test/preview-server/helpers.ts

@@ -0,0 +1,11 @@
+import {PreviewServerError} from '../../lib/preview-server/preview-error';
+
+export const expectToBePreviewServerError = (actual: PreviewServerError, status?: number, message?: string) => {
+  expect(actual).toEqual(jasmine.any(PreviewServerError));
+  if (status != null) {
+    expect(actual.status).toBe(status);
+  }
+  if (message != null) {
+    expect(actual.message).toBe(message);
+  }
+};