Supersedes #7557
It's needed to safely experiment with blueprints specifically mu-app blueprint.

There are few issues with the current confirmBlueprinted approach:

• it relies on blueprint internals:
  • it uses Blueprint files/ directory as a single source of truth. So if we have some custom logic which filters/adds some files confirmBlueprinted doesn't allow us to test this.
  • it relies on Blueprint.renamedFiles which is bad cause I believe that tests should not rely on the tested units.
• it doesn't test the file contents. Only file file paths.

I think the main issue with this approach is that if we remove half of the blueprint files then test will continue to pass.

Also there are few more concerns:

• we want to test app, addon, and mu-app w/ and w/o ember-welcome. But we don't want to have the whole project fixture for each of these cases. We need a way to use a default fixture and just apply small patches to it right in the test(w/o additional fixture folder).
• mu-app and app blueprint output have lots of common parts. It would be great not to duplicate this files. If we re-use app fixture for mu-app we can be more confident that mu-app blueprint is in sync with the app.

In order to address these issues I've introduced 2 new helpers:

loadProjectFixture(fixturePaths, options)

Loads the fixture from a single or multiple paths:

// load npm app fixture
const npmApp = options => projectFixture('app/npm', options);
// load fixtures sequentially and merge them
const yarnAddon = options => projectFixture([npmAddon(), 'addon/yarn'], options);

Under the hood it uses fixturify to load the files but the result is flattened for the sake of a better diff results.

loadProjectFixture also allows to filter result with files option which accepts minimatch patterns:

const muApp = options =>projectFixture([
  npmApp({ files: ['!app/**'] }), // load regular app but "app/" directory
  'module-unification-app/npm', // it contains only mu specific parts
], options);

And finally we can patch resulting files with a patches option:

muApp({
  patches: {
    'vendor/script.js': '// test comment',
    'package.json': function(stringContent) {
        return stringContent;
     }
  }
})

expectProject(expectedProjectName, fixture)

It's supposed to check that the current dir has an expected name and all the file content are equal to the fixture content. It prints

• a diff for each conflicting file
• list of missing and extra files if any.