Debug `sync_dry_run` flake by panicking with verbose output on failure (#13817)

Investigating #13744 

I tried reproducing here by running the test in a loop, but could not. I
presume it's an interaction with other tests.

This drops the snapshot, but I think it's worth it to try to examine the
flake?

crates/uv/tests/it/sync.rs

@@ -7879,18 +7879,13 @@ fn sync_dry_run() -> Result<()> {
      + iniconfig==2.0.0
     "###);
 
-    uv_snapshot!(context.filters(), context.sync().arg("--dry-run"), @r###"
-    success: true
-    exit_code: 0
-    ----- stdout -----
-
-    ----- stderr -----
-    Discovered existing environment at: .venv
-    Resolved 2 packages in [TIME]
-    Found up-to-date lockfile at: uv.lock
-    Audited 1 package in [TIME]
-    Would make no changes
-    "###);
+    let output = context.sync().arg("--dry-run").arg("-vv").output()?;
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        !stderr.contains("Would replace existing virtual environment"),
+        "{}",
+        stderr
+    );
 
     Ok(())
 }