Add k6-testing jslib

[oleiade]

Description

This PR adds the k6-testing library to the jslib.k6.io index.

Please fill in this template.

• 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 creates a /lib/{jslib_name} folder.
• The Pull Request creates a /lib/{jslib_name}/{desired_version} folder.
• The /lib/{jslib_name}/{desired_version}/index.js file containing the jslib's code bundle exists.
• The Pull Request updates the supported.json file to contain an entry for the newly added jslib and its {desired_version}, as in the following example:

{
  "{jslib_name}": {
    // Available package versions
    "versions": [
      "{desired_version}"
    ],

    // (optional) Documentation's or repository's URL
    "docs-url": "{documentation_or_repository_url}",

    // (optional) As a default, the homepage will point to
    // a package's bundle's index.js. If your package's main
    // bundle name is different; set it here (see the AWS
    // package for instance).
    "bundle-filename": "{index.js}"
}

• Tests have been added to /tests/basic.js and /tests/testSuite.js to ensure that the added jslib is importable and runnable by k6.