Bug 1024848 part 1 - Improve selection carets test cases

On Windows, when selecting a word by long pressing, extra spaces after
the word will also be selected. To solve this problem, I redesign the
test cases by comparing the target content with the selected content
directly while ignoring spaces at certain test cases. I believe it's
easier to understand and less error-prone than the old
replace-selected-content-and-compare way.

I added a new method SelectionManager.selected_content() to make it
easier to get the current selected text.
Bug 1024848 part 2 - Enable selection carets sanity tests on Windows browser