message: fix chatty fixtures
  A Fixtures is supposed to be able to be created once and then reused many times. And that works. But each time you use it we would rerun the create hook, causing ever more models to accumulate in the factory, causing each test to get slower than the last.
diff: @@ -4,7 +4,7 @@ import { ciSessionId } from '@cardstack/test-support/environment';
  export default class Fixtures {
  constructor({ create, destroy }) {
  - this._factory = new Factory();
  + this._factory = null;
  this._create = create;
  this._destroy = destroy;
  }
  @@ -20,7 +20,10 @@ export default class Fixtures {
  async setup() {
  if (typeof this._create !== 'function') { return; }
  + if (!this._factory) {
  + this._factory = new Factory();
  await this._create(this._factory);
  + }
  let models = this._factory.getModels();
  @@ -48,17 +51,17 @@ export default class Fixtures {
  }
  async teardown() {
  - if (typeof this._create !== 'function') { return; }
  -
  let destructionList = [];
  if (typeof this._destroy === 'function') {
  destructionList = destructionList.concat(this._destroy());
  }
  + if (this._factory) {
  let createdModels = this._factory.getModels().map(model => {
  return { id: model.id, type: model.type };
  });
  destructionList = destructionList.concat(createdModels.reverse());
  + }
  for (let [index, item] of destructionList.entries()) {
  if (!item.type) { continue; }
  