fix(tests): pass all to execa to ensure access to interleaved output

Author: dfreeman

ts/tests/helpers/skeleton-app.ts

@@ -73,7 +73,7 @@ export default class SkeletonApp {
 
   _ember(args: string[]) {
     let ember = require.resolve('ember-cli/bin/ember');
-    return execa.node(ember, args, { cwd: this.root });
+    return execa.node(ember, args, { cwd: this.root, all: true });
   }
 }