Fix duplicate test logging in --debug-rwt-logging mode in run-webkit-tests.

When in debug mode, we log every test result in the runner thread, so
the main printer should be throttled to only print periodic updates.
This was broken when I introduced the --timing flag in r151769. This
patch also adds a test to catch subsequent regressions.

R=ojan@chromium.org
BUG=260070

Review URL: https://codereview.chromium.org/20814002

git-svn-id: svn://svn.chromium.org/blink/trunk@155637 bbb929c8-8fbe-4397-9dbb-9b2b20218538