Add test to accept UTF8 characters

Make it more explicit that bad-characters contains
a unicode control character.

@YolinaS, @alexmuller
Modified key validation in backdrop to allow TE data (key accept numbers at the start, brackets, dashes and underscores)

@YolinaS
Fixed failing tests due to change in key validations.

@YolinaS
Fixed tests data according to the new regex for keys.

@YolinaS
@phss