feature(snippet): add survey snippet for some en users
Add tests for the complete sign up survey.

* A tad bit of cleanup to clarify intentions.
SURVEY_PERCENTAGE changed to `10`.

Also, the name is SURVEY_PERCENTAGE and should go from 0-100, not 0-1.
Merge pull request #1225 from mozilla/issue-1206

feature(snippet): add survey snippet for some en users