Test "add missing module (non workspace)" is very flaky

[wz1000]

It consistently fails on CI for #2789 but I can't reproduce it locally. I'm disabling it for now.

[pepeiborra]

It reproduces locally for me with #2831 - rather than flaky, it never passes.
With #2831^1 (2da5931) the test passes locally.
So it looks like #2831 has broken it.

EDIT: I was just lucky, it does pass sometimes with #2831 so it's just become flaky.