[Tests] Replay XML test files through a headless runner

Testing/README.md

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+# GUI tests
+
+Widget-level playback tests for the Cockatrice client, driven by
+[QtTesting](https://github.com/Kitware/QtTesting) (`external/qttesting`, a submodule).
+Each `*_test.xml` file is a list of events that QtTesting replays against the running
+application; a test passes when every event is handled without error.
+
+## Running
+
+```bash
+git submodule update --init external/qttesting      # once
+cmake -S . -B build/debug -DCMAKE_BUILD_TYPE=Debug -DWITH_SERVER=0 -DWITH_GUI_TEST=ON
+cmake --build build/debug -j 24
+
+./Testing/run_test.sh --all                        # headless, offscreen
+./Testing/run_test.sh Testing/smoke_test.xml       # one file
+./Testing/run_test.sh --visual Testing/smoke_test.xml   # shows the window
+```
+
+`WITH_GUI_TEST` is on by default; a checkout without the submodule configures with a
+warning and no harness. `COCKATRICE_BIN` points the runner at another build, and
+`TEST_TIMEOUT` (seconds, default 180) bounds a single test — a failing event inside a
+modal dialog can otherwise leave the process alive. The client also hard-exits a few
+seconds after playback returns, whatever the result.
+
+The tests are **not** part of `ctest`: they need a card database and a developer's
+settings, so they stay a local loop for now.
+
+## Caveats
+
+- **Not hermetic.** The client reads the local settings and card database, so a test can
+  pass on one machine and fail on another. Avoid asserting things that restored
+  workspace state or user preferences decide (for example the number of open tabs).
+- **English labels only.** Menu entries are addressed by their (translated) text, so the
+  tests expect the source language. Widgets are addressed by objectName paths, which are
+  stable across translations.
+- Actions that open a dialog are followed by a `pause` event: playback continues in the
+  dialog's own event loop, and the pause gives the UI time to build itself.
+- Widget paths include Qt-internal names where a QTabWidget is involved
+  (`qt_tabwidget_stackedwidget`), and the deck views in a game are indexed
+  (`tabbedDeckView_0`, `tabbedDeckView_1`).
+
+## Writing a test
+
+A test is a `<events>` list. Widgets are named by their path from the main window, e.g.
+`MainWindow/tabSupervisor/stackedWidget/TabGame/phasesMenu`. Useful commands:
+
+| Command | Arguments | Effect |
+|---------|-----------|--------|
+| `activate` | action or menu name | triggers a QAction (menus are opened first) |
+| `key` | Qt key code | presses that key on the focused widget |
+| `pause` | milliseconds | waits, letting the UI settle |