remote-run uses rsync instead of sftp server. Some of the tests were
incorrectly labelled as requiring sftp server. Fixed those and replaced
the requirement with rsync.
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.
