Reduce the number of messages needed to exchange buffers from 3 to 2
Move the test harnesses into test/.

This is preparation for removing the `servo-` prefixes from the various
Servo components.
Remove the `servo-` prefix from core components.
auto merge of #463 : pcwalton/servo/test-shuffle, r=metajack

This is preparation for removing the `servo-` prefixes from the various
Servo components.

r? metajack