testing story (e.g. mock shell environments)

[bobheadxi]

Some ideas:

1. encourage scripts to accept Output instances, provide outputtest package for building simple mocks
2. provide a "command factory" analogous to Buildkite's Shell, e.g. shell.Cmd() - this factory could be an interface that facilitates mocking. Alternatively, it could be shell.Run(Runner), where *Command implements Runner (now that I think of it, I like the latter):

   out := shell.Run(run.Cmd(ctx, "foobar")) // shell can mock execution and output, or propagate shared env
   

cc @jhchabran