feat(testing): add testing-helpers package for unit tests

[oscarsotoo1]

What

Add @moleculer/testing-helpers package with createBrokerHelper, mockAction and mockEvent utilities to make unit testing brokers easier.

Why

• Simplifies mocking actions/events in unit tests.
• Keeps tests isolated and readable.

Verification

• All tests pass locally: npm test ✅

Notes

• This is a self-contained package under packages/testing-helpers.
• Please run CI and let me know if any formatting/lint issues show up.

Related: moleculerjs/rfcs#3
Bounty: https://oss.issuehunt.io/r/moleculerjs/rfcs/issues/3

I claim this bounty and will maintain this PR until merged. / Reclamo este bounty y mantendré este PR hasta que sea mergeado.

[oscarsotoo1]

Hi maintainers 👋

I created this PR to add @moleculer/testing-helpers (packages/testing-helpers) which simplifies unit testing for brokers. All tests and lint pass locally (npm test, npx eslint).

I claim the IssueHunt bounty: https://oss.issuehunt.io/r/moleculerjs/rfcs/issues/3

Please review when you have a moment — I will address any feedback promptly. Thanks! 🙏