message: simplify test harness using features in ephemeral storage plugin
  ephemeral knows how to load initial models, and it performs schema validation on them to catch any errors.
diff: @@ -26,7 +26,8 @@ exports.createDefaultEnvironment = async function(projectDir, initialModels = []
  'default-data-source',
  factory.addResource('data-sources')
  .withAttributes({
  - 'source-type': '@cardstack/ephemeral'
  + 'source-type': '@cardstack/ephemeral',
  + params: { initialModels }
  })
  );
  @@ -43,21 +44,6 @@ exports.createDefaultEnvironment = async function(projectDir, initialModels = []
  disableAutomaticIndexing: true
  });
  - let writers = container.lookup('hub:writers');
  -
  - for (let model of initialModels) {
  - // TODO: this one-by-one creation is still slower than is nice for
  - // tests -- each time we write a schema model it invalidates the
  - // schema cache, which needs to get rebuilt before we can write
  - // the next one.
  - //
  - // On the other hand, this is a high-quality test of our ability
  - // to build up the entire state in the same way an end user would
  - // via JSONAPI requests. If we optimize this away, we should also
  - // add some tests like that that are similarly comprehensive.
  - await writers.create('master', session, model.type, model);
  - }
  -
  await container.lookup('hub:indexers').update({ realTime: true });
  Object.assign(container, {
  