scripting: Exit helper after running test

This avoids the helper process exiting with an error due to a broken
pipe.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1349>

data/dbus-interfaces/org.gnome.Shell.PerfHelper.xml

@@ -1,5 +1,6 @@
 <node>
   <interface name="org.gnome.Shell.PerfHelper">
+    <method name="Exit"/>
     <method name="CreateWindow">
       <arg type="i" direction="in"/>
       <arg type="i" direction="in"/>

js/ui/scripting.js

@@ -284,6 +284,15 @@ async function _runPerfScript(scriptModule, outputFile) {
         log(`Script failed: ${err}\n${err.stack}`);
         Meta.exit(Meta.ExitCode.ERROR);
     }
+
+    try {
+        const perfHelper = _getPerfHelper();
+        perfHelper.ExitSync();
+    } catch (err) {
+        log(`Failed to exit helper: ${err}\n${err.stack}`);
+        Meta.exit(Meta.ExitCode.ERROR);
+    }
+
     Meta.exit(Meta.ExitCode.SUCCESS);
 }