testscript: expose (*TestScript).stdout via SetStdout(string)

[myitcv]

(see commit message)

[mvdan]

SGTM with two nits:

• Please add a test case for a command that writes to stdout and stderr, and then returns an error. I wonder if stderr is then kept at all, for example.
• The wording for "updates the last stdout/stderr" feels slightly confusing; it feels like I could/should call it multiple times, and "last" isn't entirely correct when one is running the current command. How about "SetStdout can be used by commands in Params.Cmds to provide a resulting stdout string", since I don't believe we have another usecase yet?

[myitcv]

• Please add a test case for a command that writes to stdout and stderr, and then returns an error. I wonder if stderr is then kept at all, for example.

I've replied to this point in #139 (comment) so we keep decisions in the issue (because it feels like I'm straying from the PR being "just" the implementation here).

• The wording for "updates the last stdout/stderr" feels slightly confusing; it feels like I could/should call it multiple times, and "last" isn't entirely correct when one is running the current command. How about "SetStdout can be used by commands in Params.Cmds to provide a resulting stdout string", since I don't believe we have another usecase yet?

Will fix this as/when we agree on the API/approach.

[myitcv]

Closing in favour of #216