Add unit tests

Yay tests!

test/System/Sleep.test.cpp

@@ -0,0 +1,11 @@
+#include <SFML/System/Sleep.h>
+
+#include <catch2/catch_test_macros.hpp>
+
+TEST_CASE("[System] sfSleep")
+{       
+    const auto startTime = std::chrono::steady_clock::now();
+    sfSleep(sfMilliseconds(10));
+    const auto elapsed = std::chrono::steady_clock::now() - startTime;
+    CHECK(elapsed >= std::chrono::milliseconds(10));
+}