Simplify tests for get_(user)_template_folders
Simplify tests for get_(user)_template_folders

This will make it easier to understand the diff when we move these
tests to operate via TemplateList. Despite the verbosity, the only
attribute we were actually checking here was the name, as a way of
identifying which folders had been returned.

Looking at the three tests:

1. The first is checking we can correctly filter all folders that a
user can access, which involves appending the names of any parent
folders the user doesn't have direct access to.

2. The second is checking the same thing but also that we filter the
set of folders to just the children of a parent.

3. The third is just checking the filtering of child folders, without
any user filtering or name aggregation applied.

I've adapted tests (2) and (3) to make it clearer what is tested,
focussing the tests on a specific folder and its contents.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin