JS LPClient stub/mock needs to be polished and shared

[Gary Poster]

lib/lp/bugs/javascript/tests/test_subscription.js and lib/lp/registry/javascript/tests/test_structural_subscription.js both have some LPClient testing infrastructure repeated. It's proven to be useful for these tests. We should consider polishing this (providing clear usage comments, minimally) and extracting it to a file in something like the lp.testing.javascript YUI namespace so we follow DRY and so that others can find it, understand it, and use it.