Add dispatch tests for grid power target

[brtietz]

After we merge in the fix for #379, there will be tests for look ahead, custom and manual dispatch for behind the meter batteries. This means maintain_target is untested. Add tests with maintain_target as a dispatch option including charging and discharging under different PV and load levels.

[janinefreeman]

@brtietz I believe this dispatch method is now tested in the battery tests, isn't it?

[brtietz]

Yes! Looks like this was handled as a part of #602