Re-enable and update generator-metal-clay tests

[wincent]

These were disabled in 2017 in commit 844f56f without explanation. I want to propose a change to the generator, so must first make the tweaks necessary here to get these tests running and passing again.

I actually already submitted #1452 for this, but creating this issue retroactively to conform to the usual pattern in this repo.

[carloslancha]

Hey @wincent issues are automatically closed during release process so we keep them open, just with the label status: next-release until then. Thx!

[wincent]

Whoops, there are a couple others I closed too. I'll re-open them if you haven't already.