This was needed in an earlier version of the test, when we asserted the file
content in a more complicated way. It should have been removed in caca62b89e.
Change our fake clipboard command to not append a linefeed; that's closer to
what the production code does.
This allows us to use Equals instead of Contains for checking the clipboard
contents.
Finally, use FileSystem().FileContent() to assert the clipboard contents,
instead of selecting the clipboard file and then checking the diff view.
