This PR adds integration tests for exercising `pulumi init` and the `pulumi stack *` commands. The only functional change is merging in https://github.com/pulumi/pulumi/pull/492 , which I found while writing the tests and (of course 😁 ) wrote a regression for.
To do this I introduce a new test driver called `PulumiProgram`. This is different from the one found in the `testing/integration`package in that it doesn't try to prescribe a workflow. It really just deals in executing commands, and confirming strings are in the output.
While it doesn't hurt to have more tests for `pulumi`, my motivation here was so that I could reuse these to ensure I keep the same behavior for my pending PR that implements Cloud-enabled variants of some of these commands.
