Multi-stage example doesn't use test stage correctly

[BretFisher]

On line

docker-mastery-for-nodejs/sample-buildkit/Dockerfile

Lines 49 to 56 in 43ffcab

	## Stage 4 (testing)
	# use this in automated CI
	# it has prod and dev npm dependencies
	# In 18.09 or older builder, this will always run
	# In BuildKit, this will be skipped by default
	FROM interm as test
	CMD ["npm", "test"]

the test stage doesn't have npm dev dependencies, so any test commands would likely fail.

There are two ways to fix that:

1. Use COPY --from=dev /opt/node_modules/ /opt/node_modules/ in test stage to combine the node modules.

2. Have dev stage install all npm dependencies (remove --only=development) and then change the test stage to FROM dev as @ivorscott suggested (I like this the most).