feat(e2e): add JSON file support for extra test cases

[mdryaan]

Description

Adds a JSON loading layer to the e2e test suite so operators can inject additional foreground test cases into nerdctlTestCases() and ctrTestCases() by editing JSON files under tests/e2e/testdata/ without modifying Go source.

JSON-defined cases always use runForegroundTest (output matching via ExpectOut) since TestFunc is a function
pointer that cannot be serialized to JSON. Docker and crictl are excluded because their test runners call tc.TestFunc(tool) directly and would panic on a nil TestFunc — those can be extended in a follow-up once a function registry or similar mechanism is added.

New files:

• tests/e2e/json_test_cases.go — jsonTestCase struct,
  loadTestCasesFromJSON, optionalJSONTestCases
• tests/e2e/json_test_cases_test.go — unit tests
• tests/e2e/testdata/nerdctl_extra_test_cases.json — empty by default
• tests/e2e/testdata/ctr_extra_test_cases.json — empty by default

Modified:

• tests/e2e/test_cases.go — nerdctlTestCases() and
  ctrTestCases() now append optionalJSONTestCases(...)

Related issues

• Fixes Make end-to-end testing read from a JSON file #123

How was this tested?

Added TestLoadTestCasesFromJSON (6 subtests) and TestOptionalJSONTestCases (3 subtests) in tests/e2e/json_test_cases_test.go. All 9 subtests pass: go test -count=1 -timeout 20m ./tests/e2e/... -v -run "TestLoadTestCasesFromJSON|TestOptionalJSONTestCases" Linter passes (golangci-lint run, 0 issues). Build passes (make).

LLM usage

N/A

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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[mdryaan]

Implementation JSON test cases work for nerdctl and ctr via the foreground path. Docker and crictl are excluded for now since their runners call TestFunc directly and a nil value would panic, Happy to extend those in a follow up if needed.