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/mocha-env.mjs

@@ -0,0 +1,9 @@
+import * as chai from 'chai';
+import register from '@babel/register';
+
+// Register Babel to transpile our tests
+register({ extensions: ['.js', '.jsx', '.ts', '.tsx'] });
+
+// Set up chai.expect globally due issues with ES modules
+// See https://github.com/chaijs/chai/issues/1568
+global.expect = chai.expect;