uid-permissions test: don't assert that root can UpdateActivationEnvironment

uid-permissions test: don't assert that root can UpdateActivationEnvironment

Since 1.10.4 this is hard-coded to be disallowed when an activation
helper is used. That would be a security flaw waiting to happen,
and makes little sense anyway, because the activation helper sanitises
its environment.

Use BecomeMonitor() instead, as our way to assert that root and
messagebus are privileged.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=93036
Reviewed-by: Iain Lane