Add dummy test for snapshot_exists (#1780)

This prevents warnings about unused functions on non-illumos machines.

Author: mkeeter

integration_tests/src/lib.rs

@@ -5299,6 +5299,12 @@ mod integration_tests {
         client.detach(&volume_id.to_string()).await.unwrap();
     }
 
+    #[tokio::test]
+    async fn test_snapshot_exists() {
+        let tds = DefaultTestDownstairsSet::big(false).await.unwrap();
+        assert!(tds.snapshot_exists("foo").is_err());
+    }
+
     #[tokio::test]
     async fn test_pantry_scrub() {
         // Test scrubbing the OVMF image from a URL