Add support for multiple parallel reporters

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

tests/SelfTest/Baselines/console.std.approved.txt

@@ -1396,5 +1396,5 @@ due to unexpected exception with message:
 
 ===============================================================================
 test cases:  380 |  304 passed |  69 failed |  7 failed as expected
-assertions: 2183 | 2028 passed | 128 failed | 27 failed as expected
+assertions: 2194 | 2039 passed | 128 failed | 27 failed as expected