testauth: add a mechanism to specify groups via username
This adds a mechanism to control testauth's reported group
memberships via the username passed. You can pass a username
like 'guest:groups=foo,bar', and you will be logged in as user
'guest' with (only) the group memberships 'foo' and 'bar'. This
provides finer-grained options for testing group memberships
beyond just setting a default group membership for all users.
Signed-off-by: Adam Williamson <firstname.lastname@example.org>