The previous attempt to allow this didn't actually allow it, so this is
take two. As part of the previous attempt, I thought after tweaking the
test I had observed the test failing, and then succeeding after making
the product changes, but I must have been mistaken.
It turns out that our existing mocks tests weren't running at all
because of a missing `__init__.py` file. Once the missing `__init__.py`
is added, the tests run, but other tests ("test mode" tests) fail
because the code that creates the mocks and resources will run during
test discovery, and setting the mocks modifies global state.
To address the test issue, I've moved the mocks tests into their own
`test_with_mocks` package that can be run separately from other tests.
And addressed the original issue, by creating a root Stack resource if
one isn't already present when the mocks are set.
This change allows importing modules with calls to `pulumi.export` in unit tests. Previously, you'd have to structure the Python program in a way that avoids the `pulumi.export` from being called from unit tests.
The original version of this code caused inconsistencies in the event
loop associated with a given thread. These changes elimintate the event
loop shenanigans the mocks were trying to play by updating _sync_await
to create an event loop if none exists in the current thread.
It's possible that this will cause problems if the tests run on a
different thread than the original program, as the tests are likely to
end up waiting on outputs created by the program, which is not supported
in Python.
Also adds test coverage of the mocking/testing support in Python.
