Factor _trial_temp directory creation and cleanup so as to be re-usable by multiple runner implementations

[exarkun]

TrialRunner has various logic for selecting a _trial_temp[-N] directory, cleaning up what was there before, creating a new one, and changing the current working directory to be it.

This is probably generally useful to runner implementations. As it happens, #1784 (disttrial) introduces a new runner implementation which benefits from being able to use this code.

[exarkun]

(In [29751]) Branching to 'trial-temp-reusage-4564'

[exarkun]

Pulled the code out of the disttrial branch, cleaned it up a bit. Please review.

[exarkun]

(In [29757]) Drop this refactoring; see #4564.

[Screwtape]

Looks good to me, go ahead and merge.

[exarkun]

(In [29851]) Merge trial-temp-reusage-4564

Author: exarkun Reviewer: screwtape Fixes: #4564

Factor _trial_temp creation and cleanup code out of the runner so it can be re-used by another runner implementation.