Workaround flaky save

[apostasie]

Let's see what the CI says...

This one has been driving me bonkers for a while.

The issue is obviously still here (and I believe it is a containerd bug), but the workaround seems to work.

[apostasie]

Ok, let's not jinx it, but for me locally, it DOES fix #2327 and with four pushes, we never failed once the dreaded TestRunCustomRootfs:
https://github.com/containerd/nerdctl/actions
(other stuff failing though...)

I am convinced the underlying issue is still here though - I also reproduced it with nerdcl/mod-v2 & containerd-v2 - so, it is here to stay.

@AkihiroSuda I suggest we merge this ASAP.

Worse case scenario it does nothing (this is good hygiene anyway to have that in a separate namespace). Best case scenario, it does make our lives significantly better...

Tagging @yankay and @fahedouch - who had some discussion about the underlying issue in #2327 .

For good measure, I am namespacing as well the other save tests.

[AkihiroSuda]

Please squash the commits, then LGTM

[apostasie]

@AkihiroSuda done. Thanks.

[apostasie]

Wait. Something wrong with the project checks?

[apostasie]

@AkihiroSuda

Ok, so.
Apparently, containerd project-checks github action does bundle and depend on Vincent's https://github.com/vbatts/git-validation which by default calls git show --check which does enforce some form of linting on whitespaces.

Whitespaces "issues" have been flagged in my commit about testing from today (the ones that modified just markdown files).

The reason it did escape us is that we recently disabled running CI test for markdown-only changes...
And clearly we should still run Project Checks for these.

I'll take care of the CI right now (splitting out project checks and re-enabling them for md files changes)

Sorry about this mess.
This testing doc is cursed...

[AkihiroSuda]

Thanks