build: Add 'npm run qunit' command

build: Add 'npm run qunit' command

The running of 'grunt qunit' is unconvenient due to it only working
if the user has grunt-cli installed globally, which should not be
needed because it is already installed in the local directory.

It could be worked around by instructing users to use
`./node_modules/.bin/grunt qunit`, but it would be much simpler
to instruct them to use `npm run qunit` instead.

Unlike 'composer', 'npm' does not come by default with a command
like 'composer exec' that one could pass a command directly
without needing to register it. This is fixed in more recent
versions through 'npx -c', but that's a bit too new to require
in the manual, so adding it as a run-script instead.

Change-Id: I2812b13dbed50612b1626a617ba65f92e212f01a