gjs.tests: fix warning

The tests complained:

/nix/store/nm3nf5y4hzgmy00lw5s6ls68j38y84y0-gjs-1.72.0-installedTests/libexec/installed-tests/gjs/scripts/testCommandLineModules.sh: line 90: gjs-console: command not found

But they still passed.

nixos/tests/installed-tests/gjs.nix

@@ -3,4 +3,10 @@
 makeInstalledTest {
   tested = pkgs.gjs;
   withX11 = true;
+
+  testConfig = {
+    environment.systemPackages = [
+      pkgs.gjs
+    ];
+  };
 }