Add k6-testing jslib v0.3.0

[oleiade]

Description

• Use a meaningful title for the Pull Request. Include the name of the jslib added/modified.
• Fill the description section of the Pull Request.
• Test the change in your code, and ensure the npm run test command succeeds.
• Run yarn run generate-homepage locally and verify the new homepage /lib/index.html file looks legit.
• The Pull Request is labeled with the version bump label.
• The Pull Request adds a /lib/{jslib_name}/{desired_version} folder.
• The Pull Request adds a /lib/{jslib_name}/{desired_version}/index.js file containing the jslib's code bundle.
• The Pull Request updates the supported.json file to contain an entry for the newly added jslib version, as in the following example:

{
  "my-lib": [
    "1.0.2",
    // Use semantic versioning
    "{desired_version}"
  ]
}

• The Pull Request adds the relevant tests to the /tests/basic.js and /tests/testSuite.js files to ensure that the new version of the jslib is importable and runnable by k6.
• Merge the Pull Request once it is green. PRs adding new jslib versions do not require to get a review to be merged 🚀.