Allow capture and assertion of `.command.{err,out,log}`

[awgymer]

Currently you cannot check the stdout/stderr of a process unless it is explicitly captured as a channel.

It would be useful if nf-test could expose the content of the relevant .command.* file at e.g. process.command_out so that they can be tested without needing to be explicitly checked.

It should be possible to read and inject this information in a similar method to the way thew nextflow top level stdout is injected to e.g. pipeline.stdout

[adamrtalbot]

process.stdout and process.stderr are variables, but they always seem to be empty lists when I capture them. It could be a bug?