test(box): set uid/gid in synthetic layer tarballs (root extraction)

The oci::layers/oci::rootfs test helpers built GNU tar headers without uid/gid,
leaving those octal fields blank. A root-side extraction (which preserves
ownership, euid==0) then failed to parse the uid ("numeric field was not a
number"), so 9 tests failed on a root host (e.g. the KVM release runner) while
passing as non-root. Real OCI layers — and the production build-layer code —
always set uid/gid, so this was purely a malformed test fixture. Set uid/gid=0
in both helpers.

Author: Roy Lin

src/runtime/src/oci/layers.rs

@@ -326,6 +326,12 @@ mod tests {
             let mut header = tar::Header::new_gnu();
             header.set_size(content.len() as u64);
             header.set_mode(0o644);
+            // Set uid/gid explicitly: a bare GNU header leaves those octal fields
+            // blank, which makes a root-side extraction with preserved ownership
+            // fail to parse the uid ("numeric field was not a number"). Real OCI
+            // layers always carry valid uid/gid fields.
+            header.set_uid(0);
+            header.set_gid(0);
             header.set_cksum();
 
             builder.append_data(&mut header, name, *content).unwrap();

src/runtime/src/oci/rootfs.rs

@@ -493,6 +493,10 @@ mod tests {
             let mut header = tar::Header::new_gnu();
             header.set_size(content.len() as u64);
             header.set_mode(0o644);
+            // uid/gid must be set or a root-side ownership-preserving extraction
+            // can't parse the (blank) uid field. Real OCI layers always set them.
+            header.set_uid(0);
+            header.set_gid(0);
             header.set_cksum();
 
             builder.append_data(&mut header, filename, content).unwrap();