Add unit tests

Yay tests!

test/System/Buffer.test.cpp

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+#include <SFML/System/Buffer.h>
+
+#include <catch2/catch_test_macros.hpp>
+
+TEST_CASE("[System] sfBuffer")
+{
+    sfBuffer* buffer = sfBuffer_create();
+    CHECK(sfBuffer_getSize(buffer) == 0);
+    CHECK(sfBuffer_getData(buffer) == 0);
+    sfBuffer_destroy(buffer);
+}

test/System/Clock.test.cpp

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+#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);
+}

test/System/Sleep.test.cpp

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+#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));
+}

test/System/Time.test.cpp

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+#include <SFML/System/Time.h>
+
+#include <catch2/catch_test_macros.hpp>
+
+TEST_CASE("[System] sfTime")
+{
+    CHECK(sfTime_Zero.microseconds == 0);
+
+    sfTime time{};
+    CHECK(time.microseconds == 0);
+    CHECK(sfTime_asSeconds(time) == 0);
+    CHECK(sfTime_asMilliseconds(time) == 0);
+    CHECK(sfTime_asMicroseconds(time) == 0);
+
+    time.microseconds = 1'234'567;
+    CHECK(sfTime_asSeconds(time) == 1.234567f);
+    CHECK(sfTime_asMilliseconds(time) == 1234);
+    CHECK(sfTime_asMicroseconds(time) == 1'234'567);
+
+    CHECK(sfSeconds(0).microseconds == 0);
+    CHECK(sfMilliseconds(0).microseconds == 0);
+    CHECK(sfMicroseconds(0).microseconds == 0);
+
+    CHECK(sfSeconds(1).microseconds == 1'000'000);
+    CHECK(sfMilliseconds(1).microseconds == 1'000);
+    CHECK(sfMicroseconds(1).microseconds == 1);
+
+    CHECK(sfSeconds(10).microseconds == 10'000'000);
+    CHECK(sfMilliseconds(10).microseconds == 10'000);
+    CHECK(sfMicroseconds(10).microseconds == 10);
+}

test/System/Vector2.test.cpp

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+#include <SFML/System/Vector2.h>
+
+#include <catch2/catch_test_macros.hpp>
+
+TEST_CASE("[System] sfVector2")
+{
+    SECTION("sfVector2i")
+    {
+        const sfVector2i vector{};
+        CHECK(vector.x == 0);
+        CHECK(vector.y == 0);
+    }
+
+    SECTION("sfVector2u")
+    {
+        const sfVector2u vector{};
+        CHECK(vector.x == 0);
+        CHECK(vector.y == 0);
+    }
+
+    SECTION("sfVector2f")
+    {
+        const sfVector2f vector{};
+        CHECK(vector.x == 0);
+        CHECK(vector.y == 0);
+    }
+}

test/System/Vector3.test.cpp

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+#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);
+    }
+}