filesystem: ErrorCode::IsDirectory when opening director w/o READ

On Windows, it was possible to return a directory descriptor if READ
wasn't in the permissions.  Fixes wasmtime for
WebAssembly/wasi-testsuite#176.

Author: wingo

crates/wasi/src/filesystem.rs

@@ -1045,6 +1045,10 @@ impl Dir {
             .await?;
 
         match opened {
+            #[cfg(windows)]
+            OpenResult::Dir(dir) if !flags.contains(DescriptorFlags::READ) =>
+                Err(ErrorCode::IsDirectory),
+
             OpenResult::Dir(dir) => Ok(Descriptor::Dir(Dir::new(
                 dir,
                 self.perms,