Add a testing utility for pre-generating temp directories.
Review Request #12095 — Created Feb. 28, 2022 and submitted
TestCase.precreate_tempdirs(), which will pre-create
the specified number of temp directories, spy on
return them in order, and then immediately return them to the caller.
This allows unit tests to better work with code paths that make use of
make_tempdir(), ensuring the right results, and even pre-populating
files within them.
Made use of this in unit tests for an in-development SCM.