[WIP] Test rework permanent next-target

[apostasie]

Note:
This PR is a dump of all next- things for test rework.
It will never be merged, and will be forked-out into smaller PR.

Github actions changes:

• cosmetic efforts on test display titles (more compact, more consistent)
• lint on all platforms instead of just linux
• removal of all nick/retries
• separation of flaky / non-flaky in different steps - non-flaky now run without any retry - flaky do run with gotestsum retry, but no longer nick/retries

Integration test entrypoint:

• now lives inside ./hack/test-integration.sh
• additionally displays slow tests at the end of the run

Dockerfile changes:

• removed CMD - replaced in favor of a direct call to ./hack/test-integration.sh
• removed useless stages

Currently, test suites have been ran locally (no-flaky, on the host directly) 100 times without failure:

• ./cmd/nerdctl < 1 sec per run
• ./cmd/nerdctl/completion < 1 sec per run
• ./cmd/nerdctl/issues < 10 secs per run
• ./cmd/nerdctl/system < 10 secs per run
• ./cmd/nerdctl/volume < 10 secs per run
• ./cmd/nerdctl/network < 10 secs per run
• ./cmd/nerdctl/image < 30 secs per run
• ./cmd/nerdctl/ipfs

WORK IN PROGRESS rn:

• ./cmd/nerdctl/builder
• ./cmd/nerdctl/container

NOT part of this PR:

• ./cmd/nerdctl/login
• ./cmd/nerdctl/compose

[apostasie]

Whoever came-up with this notion that it would be cool to have the world CIs built on top of yaml + shell + a layer of crap deserves to be repeatedly whacked with a wet trout.

[apostasie]

@AkihiroSuda technically ready for review, although we should first merge the "global lock for CNI", along with "part 4" - then I will rebase and break it in cleaner commits.

[apostasie]

Has been fully subsumed into breakout PRs.