diagnostics: Don't run diagnose on disabled modules

Currently, in FreedomBox, when we go to System -> Diagnostics and run diagnostics, it runs diagnostics on all installed apps. This includes apps that are disabled. Disabled apps obviously fail the tests and show up as failed. Avoid running diagnostics on disabled modules. Just one task:

• Ensure that diagnostics tests are not run on disabled modules. There are enumeration of modules when running diagnostics on all modules. During this enumeration, we already have a condition to skip modules that are not installed. Add another condition to skip apps that are disabled. For this access the app object for the modules with module.app and check with is_enabled().

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

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