This requires a bunch of different changes across the reporter
subsystem.
* We need to handle multiple reporters and their differing
preferences in `ListeningReporter`, e.g. what to do when
we mix reporters that capture and don't capture stdout.
* We need to change how the reporter is given output and
how we parse reporter's output destination from CLI.
* Approval tests need to handle multireporter option
Martin Jeřábek