Add a number of convenience methods to test_pub.

Add a number of convenience methods to test_pub.

This changelist has two goals. The first is to simply clean up pub_test by
factoring out repeated patterns, removing the use of raw YAML, and making the
commands more semantic. The second goal is to set the stage for the use of
ephemeral ports for the mock package server, which will allow multiple instances
of the tests to be run simultaneously.

This was originally submitted as r11261, but that caused test errors due to a race condition that's fixed in this CL.

Committed: https://code.google.com/p/dart/source/detail?r=11266

[nweiz, 2012-08-23 22:21:47]

Patch set 1 contains the difference between this and the original CL.

[Bob Nystrom, 2012-08-23 22:27:14]

LGTM!