[Tests] Capture screenshots and dump the widget tree from a playback

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.

Testing/run_test.sh

@@ -12,6 +12,10 @@
 # Environment:
 #   COCKATRICE_BIN   Path to the client binary (default: build/debug/cockatrice/cockatrice,
 #                    or the .app bundle binary on macOS).
+#   COCKATRICE_SCREENSHOT_DIR
+#                    Directory the tests write screenshots and widget dumps to
+#                    (default: build/screenshots). Tests address it as
+#                    $COCKATRICE_SCREENSHOT_DIR/name.png.
 #   TEST_TIMEOUT     Seconds a single test may run before it is killed (default: 180).
 #                    Uses timeout, or gtimeout from coreutils on macOS.
 #
@@ -32,6 +36,12 @@ fi
 TEST_DIR="$REPO_ROOT/Testing"
 TEST_TIMEOUT="${TEST_TIMEOUT:-180}"
 
+# Tests that capture the UI address this as $COCKATRICE_SCREENSHOT_DIR/name.png.
+# It lives under the ignored build/ tree, so a run does not leave untracked files.
+SCREENSHOT_DIR="${COCKATRICE_SCREENSHOT_DIR:-$REPO_ROOT/build/screenshots}"
+mkdir -p "$SCREENSHOT_DIR"
+export COCKATRICE_SCREENSHOT_DIR="$SCREENSHOT_DIR"
+
 # Stock macOS has no `timeout`; coreutils provides `gtimeout`. Without either the
 # runner still works, but a hung test is not killed.
 TIMEOUT=()