Expand pkStdio, pkExec and pkSpawn utils to target executables

[tegefaulkes]

Specification

We need to expand the pkStdio, pkExec, and pkSpawn utility commands to support testing executable files such as the win, mac builds and docker container. Counterparts to these commands need to be created. These new commands need to mimic their counterparts but execute the target executable.

Additional context

Tasks

1. Create counterpart commands that execute the targeted executable
   • pkExecTarget
   • pkStdioTarget - this can't work with mocking so keep that in mind.
   • pkSpawnTarget
2. add switching utility functions that will use a pk command or it's counterpart to execute the command.
3. add code docs to each command clearly specifying usage.

[tegefaulkes]

pkXTarget commands explicitly take the command now. global.testCmd needs to be passed into it. I've also added pkXSwitch commands that will switch between the normal and target version of the pk commands. Used like so.

pkSpawn(args, env, cwd, logger);
// becomes
pkSpawnSwitch(global.testCmd)(args, env, cwd, logger);

[CMCDragonkai]

And ultimately the global.testCmd is specified through an environment variable?

Would it be possible that the test code automatically specifies the testCmd depending on the "testing platform", but that it can be overwritten by the env variable if it is set.

[tegefaulkes]

I think @emmacasolin had a way to check what platform was when tests were running. I'll have to dig through the NAT tests for that. Either way we can't do this for the docker tests.

[CMCDragonkai]

I think you should keep the entrypoint functions the same name as before. It should be just pkExec and pkSpawn as before.

But internally switch to a different implementation.

You can do this like:

function pkExec() {
  if (...) { return pkExecTarget(); }
  // ... original functionality ...
}

function pkExecTarget() {
  // new functionality
}