Properly gate symlink-related unit tests to allow local testing on Windows without Developer Mode

As per conversation with @sunfishcode just now, it seems that some WASI-related unit test failures I was seeing on MinGW and worked around in #2678 were due to a peculiarity of Windows symlink support: creating symlinks requires enabling Developer Mode, which is not the default setting (and was not enabled on my machine). There is some config plumbing in `cap-std` to determine if symlinks are supported; we should do the same in wasmtime so that developers can run the full test suite without non-default options.