Ref T13098. Since I plan to implement "send the results to Harbormaster" as another type of formatter/output/sink, just rename the objects which receive unit test results and print/write/transmit them into "Sinks" (in the sense of Source/Sink).

Get the default console sink working well enough to see what's failing. As with all other changes in this series this is very rough, but the general idea is that I want to:

• Let sinks stream both ongoing status information and final results.
• For the console output, try to increase the signal-to-noise ratio of the output stream. Today, it's too easy to lose a failed test in the results. I want to improve this by outputting less frequently and summarizing passes ("93 tests passed.") so that the streaming output mostly shows failures and it's easier to make a decision to ^C and revise if you see something you don't like.
• Also, add a summary mode at the end which makes sure failures show up on the console and aren't scrolled up 30 pages. For now, this is quite rough.