Bug 1019441 part 2 - Add marionette test cases for selection carets. r=roc, r=mdas

Add marionette test cases for selection carets feature in bug 987718.
Test cases target <input>, <textarea>, editable and non-editable
elements.

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

Run tests on b2g emulator manually:
./mach marionette-webapi layout/base/tests/marionette/test_seletioncarets.py