Support testing from within an enclave

[nick-mobilecoin]

There currently isn't a good way to unit test behavior that directly talks to the SGX SDK. While it's good practice to separate logic and dependencies, it would still be good to unit test functionality at the SGX SDK boundary.

This limitation stems from the current rust test infrastructure. The test infrastructure is invoked by the host system.
One thought is to find a way to pass in test parameters through the host into the enclave and execute that way.

graph LR
    A[untrusted side] --> |ECALL| B[trusted entry point]
    D[Test runner] --> |ECALL|E[test entry point]
    subgraph Inside Enclave
    B --> C[Code to test]
    E --> F[enclave::test]
    F --> C
    end  

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Some references:

• wasm-bindgen-test, https://rustwasm.github.io/wasm-bindgen/wasm-bindgen-test/index.html#testing-on-wasm32-unknown-unknown-with-wasm-bindgen-test
• https://ferrous-systems.com/blog/test-driver-crate/#target-testing-with-defmt-test