test: route the corecmd stub handlers through printJSON

PR #1626 landed corecmd_list_test.go / corecmd_mutation_test.go calling a writeJSON helper, green against a base that predated #1606 — which deleted writeJSON to standardize package cli on printJSON. #1626 merged two days later without a re-run against the moved base, so main's Lint (typecheck) and Tests are red and every open PR inherits the failure via the merge preview.

Point the three call sites at printJSON, the same conversion #1606 applied to the other test fixtures (repo_mirror_test.go, resolveref_test.go, ...), rather than reintroducing the deleted duplicate.