Run unit tests that require root perms in CI

[sondavidb]

Issue #, if available:
Closes #892

Description of changes:
Added the necessary flags to the Makefile to pass -test.root to the testing binaries. We can't just use GO_TEST_ARGS as passing -test.root there will cause an unrecognized flag error in all of our other packages, as only the snapshot package uses the -test.root flag, so passing it to the test binary allows all the other tests to just ignore it.

Really I don't think we should be using containerd's testutil.RequiresRoot at all. But this is a decent bandaid fix.

This also added a separate commit to fix our failing remote tests as we did not have a namespace attached to the contexts passed in which would cause them to fail.

Testing performed:
GO_TEST_BINARY_FLAGS='-test.root' make test failed locally but seems like issues with my local setup. Opening as draft to see if runners will have the same issue. If not, will take out of draft state.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sondavidb]

Been testing with go test ./snapshot -test.root with root perms.

Two issues here:

1. It seems to not be happy with the permissions I'm adding to the Mount syscalls. That's annoying and I can't figure that out...
2. The last known commit that I could get passing with these changes is ed7a741 all the way back in March 2023 😬 b64bf2f seems to be the breaking change, which is the containerd 1.7 upgrade.

What bothers me is that stargz runs this exact test but it does not fail. I attempted to run it locally and it did fail there as well. Wonder if there's some setup we're missing here...

In any case, it's an option to skip the test entirely if it's causing so much trouble. But I would like to figure it out if possible.

[sondavidb]

I found the culprit. Our CodeBuild runners are on 4.14 which is causing issues with mounting. Newest Ubuntu runners are using 6.14 which doesn't have these issues. We still have issues on CodeBuild with cross-device linking as well which I haven't figured out the reason for, but CI on regular GitHub Ubuntu runners seems to pass.

As a bandaid solution, I'll create a separate workflow that only runs root tests on an Ubuntu runner. I'll create an issue to track this as well.