Improve a bit the test suite

Hello,

This is a pull request that tries to improve a bit the current test suite:

* The `sass_project` application's `eager_load` value is not configured properly so the output of any command ran in this project will output a warning message.
* A lot of tests are relying on `runcmd` the same way so let's add a thin wrapper to DRY the tests.
* The test suite is very slow to run so let's remove calls to Bundler when running code inside the scope of an application when it's not needed.

Have a nice day.