Add a test to ensure that views data cache is cleared when flag added/deleted

To reproduce:

- start with a site with no user flags (such as on initial install of Flag)
- add a view of users
- notice there is no 'user flag' relationship, because there are no user flags yet
- add a user flag
- go back to the view and add a relationship: there is still no 'user flag' relationship, even though there is a user flag now.
- clear caches
- user flag relationship is now visible

Adding or deleting a flag should therefore clear Views data caches. For bonus points, only clear them when we're adding a flag of a type that doesn't exist yet / deleting the last flag of its type.