add failing test to expose problem with cbg data only

WIP.

@brandonarbiter @HowardLook this will contain the fix for the error when uploading CBG data only (Trello card: https://trello.com/c/lLdXaFRs), which, in a way, is kind of expected (although of course it should be dealt with more gracefully), the issue being that there are parts of tideline that require SMBG data because it's necessary for two-week view. What would you like the behavior to be if there's no SMBG data? Link to two-week view is disabled/greyed out?