We now divide all timeouts by ten for xfailing tests, with the hope to
still catch newly passing tests, but not spend too much time waiting.
With a quick test, this reduced the testsuite run length from 12 to
7-8 minutes.
Fixes#1611
This marks errors that are expected by a test with an "(Expected)"
marker and white color (instead of red).
The formatting of the log messages has been deferred to _render_log,
since the .expected attribute is not correctly set right after we read
the message.
