diagnostics: Fix IPv6 diagnostics failures

Currently, there are two IPv6 based diagnostics tests that always fail when diagnostics are from the module Diagnostics in System menu. Users are getting confused by the failing test. Fix the two issues. The following are the tests that fail:

• Listening on tcp6 port 8000. FreedomBox does not listen on IPv6 addresses and only listens on IPv4. CherryPy the web server that FreedomBox uses does not seem to have the ability to listen on IPv4 local address and IPv6 local address simultaneously. So, just remove this diagnostics test.
• Access URL http://[fe80::...%eth0]/plinth/ on tcp6. There seems to be an issue with the way curl accesses IPv6 link local address and redirections are performed. Hence, avoid redirection by using the https URL in the diagnostic test. This should fix the issue.

This issues nominated for 'Contributor Invite' (ping @dannyhaidar-guest):

• Task Area: Coding
• Effort Rating: 1/5
• Skills Required: Python