Skip the read-only-directory test where modes are not enforced

`a_failed_overwrite_puts_the_original_back` makes the presets directory
read-only and expects the overwrite to fail. CI's Desktop job runs in a
container as root, and root is not refused by a mode: the write succeeds,
the assertion fails, and build-and-test has been red on every push to
master since the test arrived.

The test now probes the refusal it depends on -- one write into the
directory it just locked -- and skips where that write goes through.
Probed rather than keyed on the uid, because what the test needs is the
refusal itself, and a filesystem mounted without permission checks would
pass a uid test and fail this one all the same.

ui/dr-ui/src/presets.rs

@@ -1608,6 +1608,22 @@ mod tests {
         perms.set_mode(0o500);
         std::fs::set_permissions(&dir, perms.clone()).unwrap();
 
+        // Root is not refused by a mode, and CI's desktop job runs as root
+        // inside its container: there the directory is as writable as it ever
+        // was and the "failed" write succeeds. Probed rather than assumed from
+        // the uid, because what the test needs is the refusal itself, and a
+        // filesystem mounted without permission checks would pass the uid
+        // test and fail this one all the same.
+        let probe = dir.join("probe");
+        let enforced = std::fs::write(&probe, b"").is_err();
+        if !enforced {
+            let _ = std::fs::remove_file(&probe);
+            perms.set_mode(0o700);
+            std::fs::set_permissions(&dir, perms).unwrap();
+            eprintln!("directory modes are not enforced here (root?); skipping");
+            return;
+        }
+
         let failed = presets.insert("Warm", Preset::default()).is_err();
 
         // Restore the permissions before asserting, so a failure here does not