Add ability to mock out process.stdout calls

Currently issueCommand always outputs to stdout here: https://github.com/actions/toolkit/blob/master/packages/core/src/command.ts#L25. This is great but when writing tests for actions it quickly fills up the logs:

> @ test /Users/njero/Code/Examples/example-github-action-typescript
> jest

 PASS  .github/actions/debug-action/__tests__/debug.test.ts
  debug action debug messages
    ✓ outputs a debug message (5ms)

##[debug]👋 Hello! You are an amazing person! 🙌
##[debug]👋 Hello! You are an amazing person! 🙌
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        1.423s
Ran all test suites.

If the output was wrapped in a call on the Command object itself then it could be mocked in the jest setup. Additionally this might pave the way for alternate logging outputs down the road.

[jasonkarns]

@ericsciple I don’t think it’s just as simple as mocking core. I’m using the tool-cache module and getting all the debug messages triggered by tool-cache. I can’t mock @actions/core because tool-cache has its own copy of the module. (@actions/core isn’t de-duped in my node_modules tree; but even if it were, that would be a pretty fragile mock to assume they would always de-dupe)

[jeffrafter]

It is also possible to mock this globally in the jest.config.js:

const processStdoutWrite = process.stdout.write.bind(process.stdout)
process.stdout.write = (str, encoding, cb) => {
  // Note: this will soon change to ::
  if (!str.match(/^\#\#/)) {
    return processStdoutWrite(str, encoding, cb)
  }
}