lisa.env: remove TestEnv

Rename lisa.env.TestEnv to lisa.target.Target. That new class is now
only used for connecting to the target, and is a thin wrapper around
devlib.target.Target.

Target.__init__ now takes plain parameters. Building from a TargetConf
is available through Target.from_conf.

Other responsabilities like creating EnergyMeter have been moved out and
can now be used independently.