test(supervisor): run sandbox instance host tests only on Unix hosts

The sandbox_instance_host tests added with per-instance supervisor
state (#4701) admit hosted instances through SandboxInstanceHost::new,
which prepares a private directory per instance. Hosted sandbox
instances are Unix-only: on other hosts prepare_private_dir returns
"hosted sandbox instances require a Unix host". Prior to this change,
the test module still compiled and ran on Windows, so all three tests
failed while creating their host.

Now, the test module is gated to Unix, and the Unix-only permission
check inside one test no longer needs its own cfg attribute.

Signed-off-by: Kris Hicks <khicks@nvidia.com>

Author: krishicks

crates/openshell-supervisor/src/sandbox_instance_host.rs

@@ -745,7 +745,8 @@ pub(crate) fn prepare_private_dir(_path: &Path) -> Result<()> {
     ))
 }
 
-#[cfg(test)]
+// Hosted sandbox instances require a Unix host.
+#[cfg(all(test, unix))]
 mod tests {
     use super::*;
     use http_body_util::BodyExt as _;
@@ -1050,7 +1051,6 @@ mod tests {
 
         // Files that cannot be deleted keep the key reserved until a later
         // removal deletes them. Root ignores the permission this relies on.
-        #[cfg(unix)]
         if !nix::unistd::geteuid().is_root() {
             use std::os::unix::fs::PermissionsExt as _;