[Tests] Capture screenshots and dump the widget tree from a playback (#7818)

pqWidgetTestEventPlayer adds the widget-level commands the UI tests were
missing:

- screenshot grabs the addressed widget and saves it, creating the parent
  directory and expanding $NAME / ${NAME} from the environment so the runner
  decides where captures land
- dumpTree writes the widget paths the tests address, for finding the path of a
  widget while writing a test
- resize sets the top-level window size, since the layout owns the children

The runner sets COCKATRICE_SCREENSHOT_DIR to build/screenshots, which is
ignored, so captures do not show up as untracked files.
Testing/screenshot_test.xml captures the main window and the settings dialog,
and Testing/README.md documents the commands, the pause/process_events split,
path drift and the capture workflow.

cockatrice/CMakeLists.txt

@@ -599,9 +599,12 @@ target_link_libraries(
 cockatrice_target_enable_crash_reporting(cockatrice)
 
 if(WITH_GUI_TEST)
-  # These two translate between XML test files and the QtTesting event pipeline;
-  # they need the QtTesting headers, so they only exist with the harness.
-  target_sources(cockatrice PRIVATE src/game_graphics/pq_cockatrice_event_player.cpp src/interface/recording.cpp)
+  # QtTesting integration and the Cockatrice-specific playback commands. They
+  # need the QtTesting headers, so they only exist with the harness.
+  target_sources(
+    cockatrice PRIVATE src/game_graphics/pq_cockatrice_event_player.cpp src/interface/pq_widget_test_event_player.cpp
+                       src/interface/recording.cpp
+  )
   target_link_libraries(cockatrice PRIVATE QtTesting)
   target_compile_definitions(cockatrice PRIVATE WITH_GUI_TEST)
 endif()