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.
We want to show an error when the user tries to invoke an action that expects only
a single item to be selected.
We're using the GetDisabledReason field to enforce this (as well as DisabledReason
on menu items).
I've created a ListControllerTrait to store some shared convenience functions for this.
