init: symlink /dev/fd to /proc/self/fd

For process substitution to work, /dev/fd needs to be symlinked to /proc/self/fd

Without the change, the newly added `test_command_process_substitution` test
would fail with:
```
---- test_command_process_substitution stdout ----
Command output=bash: line 1: cd: /tmp/.tmpr2SM0u: No such file or directory
Command output=cat: /dev/fd/63: No such file or directory
[tests/test.rs:251] get_error(recv, None) = Some(
    "Command failed with 1",
)
thread 'test_command_process_substitution' panicked at 'assertion failed: dbg!(get_error(recv, None)).is_none()', tests/test.rs:251:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:
    test_command_process_substitution

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 11 filtered out; finished in 2.41s

error: test failed, to rerun pass `--test test`
```

After the change, the test pass and it is possible to run:
```
$ cargo run --  -k $KERNEL_REPO/arch/x86_64/boot/bzImage "tail <(cat README.md)
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running `target/debug/vmtest -k /home/chantra/devel/bpf-next//arch/x86_64/boot/bzImage 'tail <(cat README.md)'`
=> bzImage
===> Booting
===> Setting up VM
===> Running command

For general architecture notes, see [architecture.md](./docs/architecture.md).

Many thanks to [`drgn`'s
vmtest](https://github.com/osandov/drgn/tree/main/vmtest) by Omar Sandoval and
Andy Lutomirski's most excellent [`virtme`](https://github.com/amluto/virtme)
for providing both ideas and technical exploration.
```

Signed-off-by: Manu Bretelle <chantr4@gmail.com>

Author: chantra

src/init/init.sh

@@ -75,6 +75,10 @@ mount -t cgroup2 -o nosuid,nodev,noexec cgroup2 /sys/fs/cgroup
 log "Mounting tmpfs at /mnt"
 mount -t tmpfs -o nosuid,nodev tmpfs /mnt
 
+# Symlink /dev/fd to /proc/self/fd so process substitution works.
+log "Symlink /dev/fd to /proc/self/fd"
+[[ -a /dev/fd ]] || ln -s /proc/self/fd /dev/fd
+
 log "Init done"
 
 # Locate our QGA virtio port

tests/test.rs

@@ -230,6 +230,32 @@ fn test_kernel_target_cwd_preserved() {
     assert_no_err!(recv);
 }
 
+#[test]
+fn test_command_process_substitution() {
+    let config = Config {
+        target: vec![Target {
+            name: "command can run process substitution".to_string(),
+            kernel: Some(asset("bzImage-v5.15-empty")),
+            kernel_args: None,
+            // `$0` is a portable way of getting the name of the shell without relying
+            // on env vars which may be propagated from the host into the guest.
+            command: "cat <(echo -n $0) > /mnt/vmtest/result".to_string(),
+            image: None,
+            uefi: false,
+            vm: VMConfig::default(),
+        }],
+    };
+    let (vmtest, dir) = setup(config, &[]);
+    let (send, recv) = channel();
+    vmtest.run_one(0, send);
+    assert_no_err!(recv);
+
+    // Check that output file contains the shell
+    let result_path = dir.path().join("result");
+    let result = fs::read_to_string(result_path).expect("Failed to read result");
+    assert_eq!(result, "bash");
+}
+
 #[test]
 fn test_qemu_error_shown() {
     let config = Config {