Adds a testing/diagtest package similar to
the previously added iotest package (#12377)
which writes log messages to a testing.TB's log stream.
This makes tests less noisy and helps associate error messages
with the exact test cases they came from.
diagtest, unlike iotest, prefixes each message with stdout or stderr.
To reuse iotest.LogWriter for this, we add support to iotest.LogWriter
to specify an optional prefix string.