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

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

Enlarge touch caret expiration time to avoid intermittent test failures.

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

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