Captures System.out/System.err streams to buffers.
Please note that this design is inherently single-threaded test-linear, and is intended only
for use with ReporterManager, which is also test-linear. While it will capture
output in a multi-threaded scenario, there's no way to associate output with the correct
test/thread.
Note; this class does not need synchronization because all of these methods are serially invoked on
the same thread. Or maybe not. See notes inside ReporterManager about the general improperness
of this design in multithreading.