[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/src/interface/pq_widget_test_event_player.h

@@ -0,0 +1,45 @@
+#ifndef PQ_WIDGET_TEST_EVENT_PLAYER_H
+#define PQ_WIDGET_TEST_EVENT_PLAYER_H
+
+#include "pqWidgetEventPlayer.h"
+
+/**
+ * Widget-level commands for the UI tests.
+ *
+ * pqCockatriceEventPlayer handles cards and zones inside the game scene. This
+ * player covers the rest of the client: capturing a widget as an image,
+ * dumping the paths QtTesting addresses widgets by, and sizing the window.
+ *
+ * Commands:
+ *   screenshot "<path>"
+ *       grabs the addressed widget and saves it as an image. Parent directories
+ *       are created, and $NAME / ${NAME} in the path expand from the
+ *       environment, so the runner decides where captures land
+ *   dumpTree "<path>"
+ *       writes the addressed widget's subtree with the paths QtTesting
+ *       addresses it by, for writing a test
+ *   resize "WIDTHxHEIGHT"
+ *       resizes the addressed widget's top-level window
+ */
+class pqWidgetTestEventPlayer : public pqWidgetEventPlayer
+{
+    Q_OBJECT
+    typedef pqWidgetEventPlayer Superclass;
+
+public:
+    explicit pqWidgetTestEventPlayer(QObject *parent = nullptr);
+
+    using Superclass::playEvent;
+
+    bool playEvent(QObject *object, const QString &command, const QString &arguments, bool &error) override;
+
+private:
+    bool playScreenshot(QObject *object, const QString &arguments);
+    bool playDumpTree(QObject *object, const QString &arguments);
+    bool playResize(QObject *object, const QString &arguments);
+
+    /** @brief Expands $NAME / ${NAME} and creates the file's parent directory. */
+    static bool resolveOutputPath(const QString &command, const QString &path, QString &resolved);
+};
+
+#endif // PQ_WIDGET_TEST_EVENT_PLAYER_H