chore: add test execution to CI pipeline

[gvc]

Summary

• Add test:coverage script to package.json
• Add nodeCommands with test step to .vtex/deployment.yaml so CI runs tests on PRs

Context

Part of test-execution rollout for Checkout Experience repos. Goal: every repo runs its test suite on every PR via DK CI.

Test plan

• Verify pipeline runs on this PR at http://dkcicd.vtex.systems/
• Verify test step executes (or stub passes if no tests)
• Confirm no regressions in existing pipeline steps

Note: Any pre-existing CI failures on this repo are unrelated to this change.

Notes for maintainer: MISSING referenceId - maintainer must add before merge;