Add SplitTextNodeCommandTest

I am planning to modify the behavior of DocumentMarkerController::copyMarkers()
(to move the markers to the new list instead of leaving pointers to the same
markers, which have possibly been modified, in the source list) and decided to
write a test to make sure we don't break SplitTextNodeCommand.

BUG=707867

Review-Url: https://codereview.chromium.org/2802993007
Cr-Commit-Position: refs/heads/master@{#463459}
3 files changed