Implement the design described in https://jira.nypl.org/browse/SIMPLY-2004 (via prototype https://invis.io/DRSFIRXY6XU )

Acceptance criteria:

1. A library user can turn on the display of test libraries on demand
2. A library user can turn off the display of test libraries on demand and without rebooting their device
3. Only a user who knows the 7-tap trick can load the list of test libraries

To test:

1. Load SimplyE on an iOS device (iOS 12+)
2. Navigate to the "Settings" screen
3. Tap the "Accounts" menu item
4. Tap the "Add Library" text on the upper right
5. Verify that "Acton Public Library" is NOT showing in the library list
6. Close library list
7. Return to "Settings" screen
8. Tap the text of the SImplyE version 7 times
9. A new menu item called "Testing" should load
10. Tap the "Testing" menu item
11. A new screen should load with a toggle button to enable test libraries and add a Custom OPDS URL.
    1. Verify the toggle starts at "off"
12. Tap the toggle to on
13. Go back to Settings
14. Tap "Add Library"
15. Verify that "Acton Public Library" now shows on the screen
16. Close SImplyE (even from running in the background)
17. Return to SimplyE
18. Navigate to Settings menu
19. Tap "Accounts" menu item
20. Tap "Add Library"
21. Verify that "Acton Public Library" remains on the list
22. Return to "Settings" menu 
23. Tap the text of the Simply E version 7 times
24. Tap "Testing" menu
25. Turn toggle off
26. Return to "Accounts" page
27. Tap "Add Library"
28. Verify that "Acton Public Library" no longer shows in the library list