Add unit tests for sakai-i18n.js

This can only be tested out by a developer. Build webcomponents, with tests on (the default). You should see output a bit like this: — frontend-maven-plugin:1.8.0:npm (JS Tests) @ webcomponents — [INFO] Running 'npm run test' in /Users/fisha/git/sakai/webcomponents/tool/src/main/frontend [INFO] [INFO] > webcomponents@1.0.0 test /Users/fisha/git/sakai/webcomponents/tool/src/main/frontend [INFO] > mocha -r esm -r jsdom-global/register -r mock-local-storage [INFO] [INFO] [INFO] [INFO] Sakai i18n [INFO] loadProperties [INFO] ✓ Expects to work with portal.locale set [INFO] ✓ Expects to work with an explicit lang [INFO] ✓ Expects properties to be in sessionStorage after the call [INFO] ✓ Expects properties to be not be in sessionStorage if caching is off [INFO] [INFO] [INFO] 4 passing (14ms)