js: more testing of nativeMessaging connection

• There were some inconsistencies between utf-8, transfer and browsers' utf16, which broke characters that were split between individual messages. src/Connection now contains a workaround that reassembles javascripts' format from passed base64 strings. This needs someone more experienced looking.
• Added several new tests which were failing during initial debugging of this issue
• reorganized BrowsertestExtension to avoid cluttering.