serve.spec.js is slow, should be rewritten to be like plugin.spec.js

The spec for `cordova serve` is slow and overly complicated.
It re-creates the project for each sub-test and and uses the vanilla platform files which causes it to hit the network if platform files are not cached in ~/.cordova/lib.

The attempt to test all platforms usable on the current box the test is running on is admirable, but it results in non-repeatability between machines which is more of a problem than benefit.