I couldn't make it work with the Babel loader. There is a possible
solution using ts-node as a loader. I didn't want to install another
package, so I chose a different workaround.
mocha-env.mjs is processed as a module thanks to the suffix, and it
sets chai. expect globally, and then I updated tests to use the global expect.
Please see https://github.com/chaijs/chai/issues/1568
[MAILPOET-6008]
