Override GDS::SSO.test_user with extra permissions

In development, the "mock bearer token" strategy used for API requests
returns a user which has only the "signin" permission by default. For 
requests to change tags to work, this user needs the "manage_tags" 
permission too. 

The easiest way to add this permission is to override the 
GDS::SSO.test_user variable with a new user that has all the necessary
permissions. The test user takes precedence over the dummy user [1] so
this user will always be selected.

A nice side effect of doing this is that it will no longer be required
to have a user in the database when running this app in development, as
the GDS::SSO.test_user field is checked first.

This file is overwritten on deploy, and GDS::SSO.test_user is not 
requested in production environments, so this will not have an effect
on the production behaviour of the app.

[1] https://github.com/alphagov/gds-sso/blob/v9.3.0/lib/gds-sso/bearer_token.rb#L49
Merge pull request #219 from alphagov/override-gds-sso-test-user

Override GDS::SSO.test_user with extra permissions