refactor(cmake): streamline GUI test suite configuration

- Updated CMakeLists.txt to always include GUI test sources when tests are built, removing the conditional check for YAZE_ENABLE_UI_TESTS.
- Added a headless test entry for the entire GUI suite, ensuring consistent execution without a GUI.

Benefits:
- Simplified configuration for GUI tests, enhancing clarity and usability.
- Improved test execution flexibility by allowing headless operation.

test/CMakeLists.txt

@@ -115,7 +115,7 @@ if(YAZE_BUILD_TESTS)
   endif()
 
   # --- Experimental & GUI Test Suites ---
-  if(YAZE_ENABLE_UI_TESTS)
+  # GUI tests always available when tests are built (uses ImGui Test Engine)
   set(GUI_TEST_SOURCES
     test_utils.cc
     e2e/framework_smoke_test.cc
@@ -132,9 +132,9 @@ if(YAZE_BUILD_TESTS)
       WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
   )
   set_tests_properties(headless_gui_suite PROPERTIES LABELS "headless_gui;experimental")
-  endif()
 
   set(EXPERIMENTAL_TEST_SOURCES
+    test_utils.cc
     integration/ai/test_ai_tile_placement.cc
     integration/ai/test_gemini_vision.cc
   )