When enabling queued tracking in plugin settings, perform the tests from Test command

[tsteur]

This way we will proactively let the user know when there are some issues. Eg we could detect whether the eval command was disabled see #24 (comment)

We won't detect all errors since some errors might occur when there is no memory left but at least we can be more or less sure the tracking will work.

If a test fails, we won't enable the queued tracking and instead show an error in the plugin settings page.

[tsteur]

Maybe we could even run a daily or weekly scheduled task, execute all the tests and if there's an error email one or all super users.

Also a system check would be useful for this