Suggestions: change how we get the display name

The function now used has internal logic for bp_disable_profile_sync(),
as well as caching.
Add missing required parameter to some tests

type was optional in an early iteration :)
Suggestions tests: use $this->create_user() to create users

factory->user->create doesn’t add xprofile data which we need for these
tests.
Whitespace tweaks
Suggestions: ID property is uppercase
Suggestions: uppercase more IDs
Suggestions: image needs to be a direct URL

get_avatar() returns a full `<img>` ta.g
phpDoc tweak
Remove Suggestions test that doesn't match API

Querying groups by slug was removed.
Create test groups from non-current user

Otherwise current user is the group admin and has access to the groups
which weren’t accounted for in the test.
Suggestions, tests: fix incorrectly named variables