Bug 960897: Add marionette test cases for touch caret, r=roc, mdas

Add marionette test cases for touch caret feature in bug 924692. Test
cases target <input>, <textarea>, and contenteditable elements.

Thanks Phoebe Chang <natsuki011077@gmail.com> for the WIP patch.
Thanks C.J. Kuo <cku@mozilla.com> for various suggestions.

Run tests on browser manually:
./mach marionette-test layout/base/tests/marionette/test_touchcaret.py