Add tests to "kernaux_printf"

.gitignore

@@ -40,4 +40,5 @@
 /tests/test_multiboot2_print
 /tests/test_multiboot2_validation
 /tests/test_pfa
+/tests/test_printf
 /tests/test_stdlib

include/kernaux/printf.h

@@ -7,7 +7,8 @@
 extern "C" {
 #endif
 
-void kernaux_printf(void (*putchar)(char), const char *format, ...);
+void kernaux_printf(void (*putchar)(char), const char *format, ...)
+__attribute__((format(printf, 2, 3)));
 void kernaux_printf_va(void (*putchar)(char), const char *format, va_list va);
 
 #ifdef __cplusplus

tests/test_printf.c

@@ -33,6 +33,11 @@ static void test(const char *const expected, const char *const format, ...)
 
 int main()
 {
+    memset(buffer, '\0', sizeof(buffer));
+    buffer_index = 0;
+    kernaux_printf(test_putchar, "Hello, World!");
+    assert(strcmp("Hello, World!", buffer) == 0);
+
     test("", "");
     test("Hello, World!", "Hello, World!");