Feature: show time of last scan

diskonaut preserves the state of the filesystem as it was when it scanned it. If the filesystem changes while the app is running (eg. someone downloads a file into the folder diskonaut previously scanned), it will not be updated.

This feature would mean adding the timestamp of the scan to the title line on the right side.

This might cause trouble with our tests (since they are snapshot tests and we can't set the timestamp in them). So when the app the tests are running, this feature should just output instead. To do this, we can use the #[cfg(not(test))] and #[cfg(test)] configurations.