compiletest: add support for unused extern notifications

As generated by --json unused-externs.

src/tools/compiletest/src/json.rs

@@ -23,6 +23,14 @@ struct ArtifactNotification {
     artifact: PathBuf,
 }
 
+#[derive(Deserialize)]
+struct UnusedExternNotification {
+    #[allow(dead_code)]
+    lint_level: String,
+    #[allow(dead_code)]
+    unused_extern_names: Vec<String>,
+}
+
 #[derive(Deserialize, Clone)]
 struct DiagnosticSpan {
     file_name: String,
@@ -113,6 +121,9 @@ pub fn extract_rendered(output: &str) -> String {
                 } else if serde_json::from_str::<ArtifactNotification>(line).is_ok() {
                     // Ignore the notification.
                     None
+                } else if serde_json::from_str::<UnusedExternNotification>(line).is_ok() {
+                    // Ignore the notification.
+                    None
                 } else {
                     print!(
                         "failed to decode compiler output as json: line: {}\noutput: {}",