Add unit tests

Yay tests!

test/System/Vector3.test.cpp

@@ -0,0 +1,13 @@
+#include <SFML/System/Vector3.h>
+
+#include <catch2/catch_test_macros.hpp>
+
+TEST_CASE("[System] sfVector3")
+{
+    SECTION("sfVector3f")
+    {
+        const sfVector3f vector{};
+        CHECK(vector.x == 0);
+        CHECK(vector.y == 0);
+    }
+}