You can't use placeholders with the hbs helper, see this issue. But, you can opt to use Ember.HTMLBars.compile. Example:

  function renderComponent(opts = {}) {
  const {
    title = 'Title',
    subTitle = 'Subtitle',
    blockContent = "{{#if (eq slot 'main')}}This is main content{{/if}}"
  } = opts;

  const content = Ember.HTMLBars.compile(`
    {{#cloud-tools-layout title="${title}" subtitle="${subtitle}"}}
      ${blockContent}
    {{/cloud-tools-layout}}
  `);

  this.render(content);
}

NOTE: You must include the ember-template-compiler in your build file in order for this to work.
ember-cli-build.js:

  if(app.env === 'test') {
  app.import('bower_components/ember/ember-template-compiler.js');
}