Add unit tests

Yay tests!

test/System/Clock.test.cpp

@@ -0,0 +1,25 @@
+#include <SFML/System/Clock.h>
+
+#include <catch2/catch_test_macros.hpp>
+
+#include <thread>
+
+TEST_CASE("[System] sfClock")
+{
+    sfClock* clock = sfClock_create();
+    
+    CHECK(sfTime_asMicroseconds(sfClock_getElapsedTime(clock)) >= 0);
+    std::this_thread::sleep_for(std::chrono::milliseconds(1));
+    CHECK(sfTime_asMicroseconds(sfClock_getElapsedTime(clock)) >= 1'000);
+
+    sfClock_restart(clock);
+    CHECK(sfTime_asMicroseconds(sfClock_getElapsedTime(clock)) >= 0);
+    std::this_thread::sleep_for(std::chrono::milliseconds(1));
+    CHECK(sfTime_asMicroseconds(sfClock_getElapsedTime(clock)) >= 1'000);
+
+    sfClock* anotherClock = sfClock_copy(clock);
+    CHECK(sfTime_asMicroseconds(sfClock_getElapsedTime(anotherClock)) >= 1'000);
+
+    sfClock_destroy(anotherClock);
+    sfClock_destroy(clock);
+}