Add test coverage for D7 vs. D8 version of behat tests

[grayside]

Let's make sure distro- and version-specific templates are properly placed into the project. For the simple distinction between D7 and D8, we can look for the string "Drupal 7" or "Drupal 8" in test/features/example.feature using assert.fileContent().

To ensure coverage is complete, we should generate at least two distinct versions of Drupal and confirm the correct template is produced.