services/device/util_darwin_test.go: devmgr multiuser tests on darwin
services/device/util_darwin_test.go: devmgr multiuser tests on darwin

Add the necessary account creation code to support running the
integration tests for the device manager on darwin on the builders in
multi-user mode.

Change-Id: Ia0d5a9e447f0aa915b039492896237dba1f123f4

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref