Before this commit, phantomjs was crashing when some code manipulates
window.getSelection (such as the composer input in discuss or in the chatter).
This commit lets us mock window.getSelection in tests that need this function
in order to work properly.
Another benefit of this commit is the possibility to simulate the state of
window.getSelection in our tests, which enables tests on the composer that
were not possible before (e.g. making sure that mentions and emojis are
correctly rendered in the input, after the user clicked on them in the dropup
menu or the bottom bar.
Vincent Schippefilt
275910e38c