Add comprehensive testing infrastructure for ROS2 Qt node

[Copilot]

This PR addresses the need for testing infrastructure in the ROS2 Qt GUI application by implementing a complete test suite covering core functionality.

Changes Made

Test Infrastructure

• Created test/ directory with three comprehensive test files:
  • test_basic_ros2.cpp - Tests ROS2 node creation, publishers, and subscribers
  • test_basic_qt.cpp - Tests Qt compilation and basic string operations
  • test_functionality.cpp - Tests OpenCV image processing and message formatting used in the application

Build Configuration

• Updated CMakeLists.txt to include proper test targets with gtest integration
• Added ament_cmake_gtest dependency to package.xml
• Added OpenCV package finding for test compilation

Development Environment

• Added .gitignore to exclude build artifacts, IDE files, and temporary files
• Created TEST_README.md with comprehensive testing instructions and documentation

Test Coverage

The tests validate:

• Basic ROS2 functionality (node creation, topic pub/sub)
• OpenCV image processing operations (cv::resize functions used in GUI)
• String formatting and message handling (publisher functionality)
• Qt compilation verification

Usage

In a ROS2 environment:

colcon build --packages-select ros2_qt_node
colcon test --packages-select ros2_qt_node
colcon test-result --verbose

The tests are designed to work without requiring a full GUI display environment while still validating the core application logic.

Fixes #1.

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