Added a list SetNew test to try and reproduce issues testing diff
customization with the Nomad provider. We are running into "diffs didn't
match during apply", with the plan diff exhibiting a strange
off-by-one-type error in a list diff:
datacenters.#: "1" => "2"
datacenters.0: "dc1" => "dc2"
datacenters.1: "" => "dc3"
datacenters.2: "" => "dc3"
The test here does not reproduce that issue, unfortunately, but should
help pinpoint the root cause through elimination.