A quick and dirty test with doctest. Should be good enough.

tests/test1.cpp

@@ -0,0 +1,11 @@
+#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
+#include "doctest.h"
+
+int factorial(int number) { return number <= 1 ? number : factorial(number - 1) * number; }
+
+TEST_CASE("testing the factorial function") {
+    CHECK(factorial(1) == 1);
+    CHECK(factorial(2) == 2);
+    CHECK(factorial(3) == 6);
+    CHECK(factorial(10) == 3628800);
+}