Make the test framework usable standalone in other projects

[apostasie]

Note this is on top of #3591, which must be merged first.

This PR makes the testing framework usable in other projects as an independent module.

Directory renaming and re-organization of the subpackage will come next.

cc @AkihiroSuda

[apostasie]

rootless v2 failure is the timeout linked to github cache outlined in #3850.

Kube failure is the usual Docker Hub 429.

rootless 1.6 fail is bizarre and a bit worrisome:

https://github.com/containerd/nerdctl/actions/runs/13462808635/job/37622050060?pr=3889#step:9:738

--- FAIL: TestRunCap/--cap-drop=all_--cap-add=NET_RAW (0.70s)
FAIL cmd/nerdctl/container.TestRunCap/--cap-drop=all_--cap-add=NET_RAW (0.70s)
=== RUN   TestRunCap
=== PAUSE TestRunCap
=== CONT  TestRunCap
    container_run_security_linux_test.go:67: allCaps=000001ffffffffff
    container_run_security_linux_test.go:48: assertion failed: error is not nil: strconv.ParseUint: parsing "": invalid syntax
--- FAIL: TestRunCap (12.96s)
panic: test executed panic(nil) or runtime.Goexit

goroutine 2480 [running]:
testing.tRunner.func1.2({0x1029160, 0x1b8cb90})
	/home/rootless/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1[734](https://github.com/containerd/nerdctl/actions/runs/13462808635/job/37622050060?pr=3889#step:9:735) +0x21c
testing.tRunner.func1()
	/home/rootless/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1737 +0x35e
runtime.Goexit()
	/home/rootless/go/pkg/mod/golang.org/[email protected]/src/runtime/panic.go:631 +0x5e
testing.(*common).FailNow(0xc0005e3180)
	/home/rootless/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1041 +0x4a
gotest.tools/v3/assert.NilError({0x1380b10, 0xc0005e3180}, {0x1375560, 0xc0003eb290}, {0x0, 0x0, 0x0})
	/home/rootless/go/pkg/mod/gotest.tools/[email protected]/assert/assert.go:177 +0xee
github.com/containerd/nerdctl/v2/cmd/nerdctl/container.getCapEff(0xc00034a000, {0xc000058dc0?, 0x2?, 0x61?})
	/go/src/github.com/containerd/nerdctl/cmd/nerdctl/container/container_run_security_linux_test.go:48 +0x52c
github.com/containerd/nerdctl/v2/cmd/nerdctl/container.TestRunCap.func1(0xc00044fc00)
	/go/src/github.com/containerd/nerdctl/cmd/nerdctl/container/container_run_security_linux_test.go:114 +0x6a
testing.tRunner(0xc00044fc00, 0xc00049ad20)
	/home/rootless/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1792 +0xf4
created by testing.(*T).Run in goroutine 1694
	/home/rootless/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1851 +0x413
FAIL cmd/nerdctl/container.TestRunCap (12.96s)

[apostasie]

Rebasing to trigger a new CI run.

[apostasie]

Rebased now that the PTY PR is merged.

[apostasie]

It is 2025, and CI is still as effed up as in 1995.

This is disheartening.

• build failed because ubuntu servers connectivity: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/noble-updates/universe/binary-arm64/Packages.gz
• docker fail is recent regression CI: tests sending signals to be caught by in-container shell scripts are racy #3908
• all the others are failing because of recent github cache sluggishness

[apostasie]

Force-pushing amended commit message to trigger a CI re-run.