js: fixing Key import/export test

• BrowserTestExtension:
  • The KeyImport/Export test had some errors, which have now been fixed
  • The secret key used for the test examples is now placed more prominently, and a clarification added that decrypt tests will not work if this key is not imported.

• permittedOperations.js: typo
  
  Thanks to rrenkert@intevation.de for the fixes