Add test infrastructure for core commands (e.g. `addUri`)

[magnus-madsen]

It would be nice if we could (somehow) test (some of):

apiAddUri = 'api/addUri',
apiRemUri = 'api/remUri',
apiAddPkg = 'api/addPkg',
apiRemPkg = 'api/remPkg',
apiAddJar = 'api/addJar',
apiRemJar = 'api/remJar',

This will give us confidence as we refactor.