wasi:filesystem: Add test that one can't dot-dot out of a child dir

I would love for this to be otherwise, but
WebAssembly/WASI#856.  Also, I would like to
test symlinks, but
bytecodealliance/wasmtime#12172.

Author: wingo

tests/rust/wasm32-wasip3/src/bin/filesystem-dotdot.json

@@ -0,0 +1,3 @@
+{
+    "dirs": ["fs-tests.dir"]
+}

tests/rust/wasm32-wasip3/src/bin/filesystem-dotdot.rs

@@ -0,0 +1,99 @@
+use std::process;
+extern crate wit_bindgen;
+
+wit_bindgen::generate!({
+    inline: r"
+  package test:test;
+
+  world test {
+      include wasi:filesystem/imports@0.3.0-rc-2025-09-16;
+      include wasi:cli/command@0.3.0-rc-2025-09-16;
+  }
+",
+    additional_derives: [PartialEq, Eq, Hash, Clone],
+    // Work around https://github.com/bytecodealliance/wasm-tools/issues/2285.
+    features:["clocks-timezone"],
+    generate_all
+});
+
+use wasi::filesystem::types::{DescriptorFlags, ErrorCode, OpenFlags, PathFlags};
+
+async fn test_symlink() {
+    match &wasi::filesystem::preopens::get_directories()[..] {
+        [(dir, _)] => {
+            dir.create_directory_at("child.cleanup".to_string())
+                .await
+                .unwrap();
+            let child = dir
+                .open_at(
+                    PathFlags::empty(),
+                    "child.cleanup".to_string(),
+                    OpenFlags::DIRECTORY,
+                    DescriptorFlags::MUTATE_DIRECTORY,
+                )
+                .await
+                .unwrap();
+
+            drop(
+                dir.open_at(
+                    PathFlags::empty(),
+                    "x.cleanup".to_string(),
+                    OpenFlags::CREATE | OpenFlags::EXCLUSIVE,
+                    DescriptorFlags::READ,
+                )
+                .await
+                .unwrap(),
+            );
+
+            drop(
+                dir.open_at(
+                    PathFlags::empty(),
+                    "x.cleanup".to_string(),
+                    OpenFlags::empty(),
+                    DescriptorFlags::READ,
+                )
+                .await
+                .unwrap(),
+            );
+
+            // Possibly revisit if
+            // https://github.com/WebAssembly/WASI/issues/856 ever
+            // changes things.
+            assert_eq!(
+                child
+                    .open_at(
+                        PathFlags::empty(),
+                        "../x.cleanup".to_string(),
+                        OpenFlags::empty(),
+                        DescriptorFlags::READ
+                    )
+                    .await
+                    .unwrap_err(),
+                ErrorCode::NotPermitted
+            );
+
+            drop(child);
+            dir.remove_directory_at("child.cleanup".to_string())
+                .await
+                .unwrap();
+            dir.unlink_file_at("x.cleanup".to_string()).await.unwrap();
+        }
+        [..] => {
+            eprintln!("usage: run with one open dir");
+            process::exit(1)
+        }
+    }
+}
+
+struct Component;
+export!(Component);
+impl exports::wasi::cli::run::Guest for Component {
+    async fn run() -> Result<(), ()> {
+        test_symlink().await;
+        Ok(())
+    }
+}
+
+fn main() {
+    unreachable!("main is a stub");
+}