Add testing infrastructure to the standard library

[matthew-healy]

Is your feature request related to a problem? Please describe.

Currently there's no recommended/built-in way to write tests for Nickel code. It's not particularly hard to do (see the simple testing infra in tests/integration/pass/lib/assert.ncl for an example), but it seems like something enough projects will want that it's worth considering it for inclusion in the stdlib.

Describe the solution you'd like

It's probably sufficient to have some simple assertion primitives (in the form of contracts, I guess). It might be nice to spike out some ideas for more opinionated test structures, but I guess there'd be a higher bar there for inclusion in the stdlib.

Describe alternatives you've considered

The main alternative is to provide testing infrastructure in its own library. I'm not against this in principle, but given we don't have a simple way of importing Nickel packages yet (and we're unlikely to have one in the near future) I think it makes sense to at least add something official to the stdlib.

[vkleen]

A minimal step towards this could be putting Assert and check from tests/integration/pass/lib/assert.ncl into a stdlib namespace, e.g. std.tests. That seems to be all the Nickel specific functionality we have needed for a while; later we could maybe add some automation into the CLI to make it simpler to hook up.

[matthew-healy]

Yeah, I agree that could be a reasonable first step. Though since stdlib releases are currently tightly coupled to language releases I wonder if we should explore the design space here a little more before making a final decision.