Add unit tests

Yay tests!

test/CMakeLists.txt

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+include(FetchContent)
+
+set(CATCH_CONFIG_FAST_COMPILE ON CACHE BOOL "")
+set(CATCH_CONFIG_NO_EXPERIMENTAL_STATIC_ANALYSIS_SUPPORT ON CACHE BOOL "")
+FetchContent_Declare(Catch2
+    GIT_REPOSITORY https://github.com/catchorg/Catch2.git
+    GIT_TAG v3.5.3
+    GIT_SHALLOW ON)
+FetchContent_MakeAvailable(Catch2)
+include(Catch)
+
+# Build Catch2 in C++17 mode to enable C++17 features
+target_compile_features(Catch2 PRIVATE cxx_std_17)
+
+# Ensure that Catch2 sources and headers are not analyzed by any tools
+set_target_properties(Catch2 PROPERTIES COMPILE_OPTIONS "" EXPORT_COMPILE_COMMANDS OFF)
+set_target_properties(Catch2WithMain PROPERTIES EXPORT_COMPILE_COMMANDS OFF)
+get_target_property(CATCH2_INCLUDE_DIRS Catch2 INTERFACE_INCLUDE_DIRECTORIES)
+target_include_directories(Catch2 SYSTEM INTERFACE ${CATCH2_INCLUDE_DIRS})
+
+add_executable(test-csfml-system
+    System/Buffer.test.cpp
+    System/Clock.test.cpp
+    System/Sleep.test.cpp
+    System/Time.test.cpp
+    System/Vector2.test.cpp
+    System/Vector3.test.cpp
+)
+target_link_libraries(test-csfml-system PRIVATE csfml-system Catch2::Catch2WithMain)
+set_target_warnings(test-csfml-system)
+catch_discover_tests(test-csfml-system)
+
+# Copy DLLs into the same directory
+if(SFML_OS_WINDOWS AND NOT CSFML_LINK_SFML_STATICALLY)
+    add_custom_command(TARGET test-csfml-system PRE_BUILD COMMAND ${CMAKE_COMMAND} -E copy $<TARGET_RUNTIME_DLLS:test-csfml-system> $<TARGET_FILE_DIR:test-csfml-system> COMMAND_EXPAND_LISTS)
+endif()