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
in test cases which need to move touch caret.

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