What does this PR do?

Adds the tests from serverless-tools to the test stage of the pipeline for this project.

Motivation

This PR #689 made a change to our build script but we forgot to make the corresponding change in serverless-tools. That led to all PRs failing on dd-trace-js.

By adding the serverless-tools tests to this repo's pipeline, we will be ensured that does not happen again.

Testing Guidelines

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)