Add symlink permission detection to testsuite driver on Windows?

Currently the testsuite driver naively assumes that symlinks are unusable on Windows unless the FORCE_SYMLINKS environment variable is set. We set FORCE_SYMLINKS in our Windows CI configuration but then later (perhaps due to an update) found that the symlink permissions had been revoked on some builders, resulting in hard-to-diagnose CI failures.

It would be better if the testsuite driver could discover on its own whether symlinks are usable.