Add unit tests

Yay tests!

test/System/Time.test.cpp

@@ -0,0 +1,31 @@
+#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);
+}