test: cover the driver, transport, and container logic that had none

[tas50]

Line coverage of lib/ was 85.3%, and the gaps were not obscure corners.

What was untested

Area	Why it matters
Driver#default_image, #default_platform	This is what makes "a platform name and nothing else" work, which is the first thing the README promises. Nothing asserted that ubuntu-24.04 becomes ubuntu:24.04, or that centos-7 becomes centos:centos7.
The driver's lazy default_config blocks	image, platform, username, run_command, build_context, instance_name, package_name, socket.
Container#hostname	Three branches — local, remote socket, internal network — all of which produce something that looks like a host, so getting it wrong fails later as a connection timeout rather than here.
Container#upload, Container::Linux#create	create populates the state the transport and verifier read; a key missing from it surfaces as a nil far from the cause.
Every public method on the transport	#connection, Connection#execute, #upload, #container, and the temp_dir / username / socket defaults.
Driver#create, #destroy, #container, #wait_for_transport, #verify_dependencies

A spec that was testing itself

describe "socket default config logic" re-implemented the driver's default_config :socket block inside the spec file and asserted on its own copy:

def resolve_socket
  socket = "unix:///var/run/docker.sock"
  socket = "npipe:////./pipe/docker_engine" if Gem.win_platform?
  ENV["DOCKER_HOST"] || socket
end

That passes with the driver's block deleted. It now reads the value back off a driver, which is what Test Kitchen does.

Result

lib/kitchen/docker/container.rb              71.8% -> 100.0%
lib/kitchen/docker/container/linux.rb        88.8% -> 100.0%
lib/kitchen/driver/docker.rb                 78.8% -> 100.0%
lib/kitchen/transport/docker.rb              72.0% -> 100.0%
TOTAL                                        85.3% ->  94.5%

(measured with SimpleCov, which is not added to the Gemfile — it was only used to find the gaps)

The remaining gaps are Container::Windows#execute and three ContainerHelper methods, which #496 covers, and the InSpec verifier patches, which only run when kitchen-inspec is in the bundle — CI unit tests run with BUNDLE_WITHOUT=development, so they are not loaded there.

No production code is changed.

Verification

$ bundle exec rake test
416 examples, 0 failures

$ cookstyle --chefstyle          # Cookstyle 9.0.0 / RuboCop 1.90.0
44 files inspected, no offenses detected