Add test for validation of blueprint yaml files

[simonihmig]

As suggested in #7372 a validation test could detect invalid yaml files emitted by ember-cli, such as the syntax error introduced in #7349 to .travis.yml.

Besides generic yaml syntax checking, the TravisCI Linter could provide additional Travis-specific linting.

[locks]

This sounds generally like a good improvement! Of course nowadays we would need to validate the GitHub Action instead of the Travis CI file.