This introduces TestCase.precreate_tempdirs(), which will pre-create

the specified number of temp directories, spy on make_tempdir() to

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.