Auto-save test code

[ceball]

When you edit a cell, you don't have to click save. If you edit a test, then click away without saving, your test is lost :(

[timkpaine]

you actually do have to save if you edit a cell, unless the autosave picks it up. if you edit a cell and ctrl-r the page, your edits will be gone. but we should support ctrl-s shortcut and we can setup a timer to periodically save the tests to the notebook (where they'll be picked up by autosave).

[ceball]

Ah, I wasn't clear. It's worse than that :)

If you edit a cell, and then you go click in some other cell, your edits in the first cell aren't lost. (Yes, ultimately you have to save the notebook, but that's not what I'm talking about.)

If you edit a test, then you click in some other cell (e.g. to edit another test), your edits to the first test are immediately lost (if you didn't click save in the test editor).