FEAT: add an optional `stdin: string` parameter to `execute`

[vassudanagunta]

A lot of CLI commands follow the Unix philosophy and read stdin for default input and write output to stdout by default.

cli-testing-library already makes it easy to test the latter case as the execute API provides access stdout and stderr. But to test a simple CLI command that reads stdin, one is forced to use the spawn API. This is ok, but it would very nice if an optional stdin: string parameter were supported as follows:

it('program runs successfully', async () => {
    const { execute, cleanup } = await prepareEnvironment()
    const { code, stdout, stderr } = await execute(
        'node',
        './cli.js',
        'Hello? (Hello, hello, hello) / Is there anybody in there?'
    )
    expect(stdout).toBe(['Just nod if you can hear me / Is there anyone home?'])
    await cleanup()
});

I know this would clash with the existing optional runFrom parameter. Since your are in early alpha now might be a good time to switch a different way for users to pass in optional params, e.g. an options object, e.g.:

const { code, stdout, stderr } = await execute(
    'node',
    './cli.js',
    {stdin: 'Hello? (Hello, hello, hello) / Is there anybody in there?',
     runFrom: './testContext'}
)

Another alternative:

const { code, stdin, stdout, stderr } = await execute(
    'node',
    './cli.js'
)

// writes to and immediately closes stdin:
stdin('Hello? (Hello, hello, hello) / Is there anybody in there?')