Give the video effects more room in the Add effect menu

With eight video effects their list filled its 110-pixel cap, and the menu's
headings take more than the 120 pixels the height test allowed for them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/gui_tests.cpp

@@ -242,7 +242,9 @@ void runTests(Gtk::Window& window) {
     }
     check(scrolled, "the image and the video entries scroll, each on their own");
     check(cap > 0 && cap <= 480, "past a height that fits a small screen");
-    check(menu->get_height() > 0 && menu->get_height() <= cap + 120,
+    // The rest is the two headings, the separator and, in a still, the hint
+    // under the video heading.
+    check(menu->get_height() > 0 && menu->get_height() <= cap + 170,
           "and the menu stays within it however many effects there are");
     add->popdown();
     pump();