This was broken by change 264294
as lineNumButton is inside of lineNum, the `user-select` should
still on lineNum (the `td` element) instead of the button.
Same happens to coverage, lineNum is the lineElement, coverage
class was added to it instead of the button inside.
Added a regression test on `user-select` for diff line.
Change-Id: Ie40fa3ce9451fe58c3c4c2b5b14bfe13d61bceab