feat: testing isolation

[davidcavazos]

Description

This is an effort to speed up checks for pull requests, while at the same time simplifying the existing testing infrastructure in Node.js by using a dynamic strategy matrix.

Note: We're currently not using the run-all command, that's infrastructure that will eventually be used to optimize the nightly jobs.

These new tests are marked as experimental, so they shouldn't block any pull requests from merging. Once we have all the tests working on the new infrastructure and we feel comfortable with it, we can go ahead and disable the old tests.

Checklist

• [x] I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• [x] Tests pass: npm test (see Testing)
• [x] Lint pass: npm run lint (see Style)
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [x] This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [ ] Please merge this PR for me once it is approved