* Test the Python language host end-to-end
This commit introduces an end-to-end language host testing framework for
the Python SDK, similar to what already exists for the Node SDK. The
real language host is used to run Pulumi programs written in Python
while mocking out the resource monitor.
* Add new tests
* Print out better diagnostics when the langhost fails to launch
* Use the in-tree executor for testing
* CR: Place tests and code being tested in the same directory for ease of understanding, add a README
* Turns out I misunderstood the semantics of resource registration - fix two tests so that they pass now and fix a few bugs in the test harness
