feat: add testing helpers and instructions

[matthewrobertson]

This commit adds some helpers for unit testing cloud functions along with some documentation about how to use them. Much of inspiration came from the Ruby Functions Framework: https://github.com/GoogleCloudPlatform/functions-framework-ruby/blob/main/docs/testing-functions.md

A readable preview of the docs is here.

Eventually we will want to update this guide on DevSite to match the guidance in the docs we are adding in this PR; however, at the moment none of our public docs or samples are using the declarative function signature APIs so the helpers / instructions here are not compatible. Once the majority of the nodejs public documentation is updated to use the declarative APIs we can update the testing guide.

I also refactored some of our out tests to use the helpers.

Fixes #387

[grant]

It would help to have more details as to how this interface is expected to be used here. This utility is useful.

This PR is large, substantial, and external-facing. It includes both implementation and documentation. Here's an example PR description that would help:

---

feat: add testing helpers and instructions

This PR blocks https://github.com/GoogleCloudPlatform/nodejs-docs-samples/pull/2428

Feat:

• Adds a @google-cloud/functions-framework/testing package
  • Includes X, Y, Z interfaces
• This testing utility is one-off for the Node FF

Docs:

• Adds a guide on testing functions
  • sample for loading, registering, and running tests using assert
  • testing HTTP functions
  • testing CloudEvent functions

This content should not go in our docs because of X, Y:

• https://cloud.google.com/functions/docs/testing/test-http
• https://cloud.google.com/functions/docs/testing/test-event

I plan to write this testing guide for Node.js only. We'll follow-up with each language individually.

[matthewrobertson]

We typically defer heavier documentation to DevSite docs as there can be a mix of guidance.

@grant currently none of our Node.js docs on DevSite are using the declarative function signature APIs, so this guide and the testing module are incompatible with those docs / samples. Once we update all (most?) of the other docs on DevSet to use declarative I think we should update this guide to align it with the docs and helpers I am adding here. The plan for that is still TBD so I think checking some docs in here is a good interim solution.

Also FWIW the dotnet and ruby repos already include similar markdown docs that cover testing functions.