what Lori said, also your yard and sprinkler can provide all of the testing you will need to get started, and your shower. Put on your boots and rain gear and stand in the shower for 20 minutes - still dry underneath? Then try taking off that wet geat and getting into a bivy in the front yard with your sprinklers going, and finally try spending a hour in there with water beating on you and imagine getting zero sleep and then get up and put those clothes on, again with the sprinkler on. You simply cannot change clothes in many bivies and small tents without getting yourself and all of your gear wet. Providing a dry place for your gear (and you) is the purpose of "shelter", so take one that will work under the possible scenario.
With a good shell on your sleeping bag, a bivy is not required
With a large pack a stuff sack is not required, and with a large enough stuff sack a pack is not required.
You must also consider bugs, don't for get a bug net - atleast a neadnet.