Mock servers shouldn't pollute test output.

BY writing to stderr, BaseHTTPRequestHandler writes log messages to the
console during testing.  This makes the output harder to interpret.
Write the log messages to stdout instead, so that test runners will
suppress them during passing tests, and show them during failing tests.

It would be nice to have a place to write this method just once for the
Youtube and LTI mock servers, but we don't seem to have a place for code
as common as that.