test and rework: Added push test and cahnged arena_push return

So added a little test that just adds 3 ints to an arena, nothing much
but the basics work, also changed arena_push because it only pushed and
didn't returned a ponter (wich is fucking useless) so also fixed that,
everything seems fine now, need more tests.

# Tipos:
# feat, fix, refactor, docs, style, test, chore

test/test_arena.c

@@ -1,6 +1,42 @@
-#include "arena.h"
+#include <stdalign.h>
 #include <stdlib.h>
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <setjmp.h>
+#include <cmocka.h>
+
+#include "arena.h"
+
+static void test_push_3_ints(void **state) {
+    (void) state;
+
+    ArenaResult value = arena_init(sizeof(int) * 3);
+    assert_true(value.is_valid);
+    Arena arena = value.arena;
+
+    int int_to_push = 20;
+    ArenaPointer result = arena_push(&arena, &int_to_push, sizeof(int), alignof(int));
+    assert_true(result.is_valid);
+    assert_int_equal(20, (int) *result.address);
+
+    int_to_push = 30;
+    result = arena_push(&arena, &int_to_push, sizeof(int), alignof(int));
+    assert_true(result.is_valid);
+    assert_int_equal(30, (int) *result.address);
+
+    int_to_push = 40;
+    result = arena_push(&arena, &int_to_push, sizeof(int), alignof(int));
+    assert_true(result.is_valid);
+    assert_int_equal(40, (int) *result.address);
+}
 
 int main(void) {
-    return EXIT_SUCCESS;
+    const struct CMUnitTest tests[] = {
+        cmocka_unit_test(test_push_3_ints),
+    };
+
+
+    return cmocka_run_group_tests(tests, NULL, NULL);
 }