Workaround for the issue with chai v5 and ES modules

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]

mailpoet/tests/javascript/utilities/date.spec.ts

@@ -1,4 +1,3 @@
-import { expect } from 'chai';
 import moment, { Moment } from 'moment';
 import { MailPoetDate } from '../../../assets/js/src/date';