We struggled with the same thing - shared tests, as that is the only real thing you start missing when you switch from serverspec to inspec (well and also audit mode). In interacting with Chef support around this issue we discovered that there currently is no knife command and it doesn't work with enterprise Github, private repos or repos that require ssh keys. The inspec folks are working on a new specification on this as Irving points around this.
Instead we ended up setting up a compliance server for this purpose, as it has a cli for publishing profiles via "inspec compliance upload". I set up a system to be able to spin up compliance servers on demand and publish on source change for sharing. However I would have preferred to do this all through the supermarket, as now I have to manage another server and publish to another place for a similar function.
We put up with needing dedicated repos for the shared tests - our devs didn't like that as they were used to having shared tests in their cookbook repos with serverspec/audit mode. It was an easy sell though because inspec is so much faster and has so much more functionality.
We've discovered that having a compliance server is the best way right now, as not only do we get shared tests but it is also a supported scenario by the audit cookbook https://github.com/chef-cookbooks/audit - so we have everything we had before with serverspec/audit mode - it's just a lot faster now.
+1 to supporting this use case via the supermarket though, as it was a tough sell at first to justify why we needed so many different servers just to do the same thing we did before with just one server.