mgr/dashboard: reset login to admin user

mgr/dashboard: reset login to admin user

After logging in with a separate testuser, especially a read-only
user, we need to reset the login to the admin user. This is important
in order to be able to continue with further tests. If not doing so
further tests might fail with the following error response:
{"status": "401 Unauthorized",
 "detail": "You are not authorized to access that resource", ...}

Fixes: https://tracker.ceph.com/issues/42226,
https://tracker.ceph.com/issues/42227
Signed-off-by: Tatjana Dehler <tdehler@suse.com>