tests: Skip image.rs test for s390x

There is an issue on pulling an image `mcr.microsoft.com/hello-world` on s390x.
It looks a load balancer for the microsoft registry is unstable so that the runner
was able to pull the image with 10& success ratio (see
confidential-containers/trustee#417)
It is not reasonable to let the test run under the unstable environment.

This commit skips the tests at image.rs for the platform.

Signed-off-by: Hyounggyu Choi <[email protected]>

image-rs/src/image.rs

@@ -510,6 +510,7 @@ fn create_bundle(
     Ok(image_id)
 }
 
+#[cfg(not(target_arch = "s390x"))]
 #[cfg(feature = "snapshot-overlayfs")]
 #[cfg(test)]
 mod tests {