Add framework for writing unit tests

[ma2bd]

Motivation

It should be straight forward for developers of user applications to be able to write unit tests for their applications. Ideally, it should use the same test infrastructure used for normal application development.

[ma2bd]

See also near/near-sdk-rs#467

[jvff]

See also near/near-sdk-rs#467

I started looking into this, and it seemed like the solution they reached was to write a custom procedural macro for tests (webassembly-test) and a custom test runner (webassembly-test-runner), as per this comment.

This seems like something we can do. I wrote some trivial tests for the counter example application and tried to run them using webassembly-test-runner. Unfortunately it didn't work because the WASM runtime says that it can't run the module without the system API exports it depends on. I started changing the test-runner to export the necessary functions, but I haven't finished yet.