Expose API like pulumi.runtime.setMocks to mock Config and StackReferences

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Issue details

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Improve upon the unit testing experience in Pulumi by exposing an API similar to pulumi.runtime.setMocks to mock Config and StackReferences.

Additional Details

While it is possible to mock these resources today, it isn't easy. There has been some previous discussion about this experience here as well as overall testing experience feedback in other issues.

Highlighted pain points

• You can mock calls to newResource and call via pulumi.runtime, but not StackReferences and Config objects. These are required to be mocked or the test will not conclude.
• Mocking library choices can cause many differences in implementation
• You need a stubbing library to accomplish the mocking.
  • Must return from stubbed functions
  • Pass the correct nesting level of stubbed functions
  • Verify that stubbed functions are being called

Affected area/feature

Testing SDK

Related issues

#10975
#4472
#9212