How I write "contract tests"

This comes up in conversation often enough that I want to write it down..

Context:

My code talks to an external dependency that is awkward to use in unit tests.

I can refactor most of my code to eliminate the dependency. (See DEP and Whole Value). But I still have some code that talks to the external dependency. I wrap the dependency with an adapter (see Ports-and-Adapters) of significant thickness and abstraction (see Mimic Adapter). In test, I replace the real dependency with legitimate, but simplified test double (see Simulators). 

Problem:

I can't be certain that my simulator has fidelity with my real system. They may behave differently, allowing my tests to pass when my system has a bug. (This is a common problem with mocks.)

Solution:

Write one set of tests for the port, running the tests against both the real and simulated implementation.

In C#:

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Tests on the simulator are fast enough to run with every build.

Tests on the real system may be slow; they may require awkward setup; they may cost real dollars to run. You may decide to run them only in your CI or once per sprint or whatever. Since adapters are relatively stable, that can be OK.