Not only does this give us a huge speed-up, it also works around a problem
where sftp doesn't always update the modification time, which causes random
test failures.
rdar://88179140
Not all the tests are possible to run on Windows since they expect a
Unix-like shell environment. However, the rest of the tests can be
accommodated. This actually found an issue in the implementation.
Since the implementation assumes the target is POSIX/Unix-y, we should
use posixpath rather than os.path which uses the host's path style.
Really we ought to clear out "%t*", but running wildcards with 'rm'
scares me a little too much for that. This still fixes a handful of
tests that were relying on %t being an empty directory.
Note that this doesn't interfere with generated files persisting
between %target-run invocations; they'll be downloaded and then
re-uploaded as long as they're mentioned in the invocation.
