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.
This is mostly important to allow remote-run to be used to upload
files generally, while still using the same interface and arguments
that will be used to actually run executables.
(instead of locally)
The primary use case I'm looking at is easier testing across macOS
versions, but it could also be used for cross-compilation tests in
general (someday).
