tests: Add test running basic perf test

This test runs all of GNOME Shell using the headless backend inside a
mocked D-Bus environment. The basic test tests, well, basic things, like
the panel menu, the overview, showing the app grid view, as well as
going back to the session view.

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

js/ui/scripting.js

@@ -77,7 +77,7 @@ function _getPerfHelper() {
 }
 
 function _spawnPerfHelper() {
-    let path = Config.LIBEXECDIR;
+    let path = GLib.getenv('GNOME_SHELL_BUILDDIR') || Config.LIBEXECDIR;
     let command = `${path}/gnome-shell-perf-helper`;
     Util.trySpawnCommandLine(command);
 }