[ticket/11859] Make avatar drivers return template filename

The service name might not follow the expected naming scheme which would
cause abnormally long filenames and confusion for authors that might add
more avatar drivers.

PHPBB3-11859
[ticket/11859] Add missing get_template_name method to test drivers

PHPBB3-11859
Merge remote-tracking branch 'Marc/ticket/11859' into develop

* Marc/ticket/11859:
  [ticket/11859] Add missing get_template_name method to test drivers
  [ticket/11859] Make avatar drivers return template filename