test: hashmap - cripple the hash function by truncating the input rather than the output

The reason for the crippled hash function is to reduce the distribution
of the hash function, do this by truncating the domain rather than the
range. This does introduce a change in behavoir as the range is no longer
contiguous, which greatly reduces collisions.

This is needed as a follow-up patch will no longer allow individual hash
functions to alter the output directly.

src/test/test-hashmap-plain.c

@@ -693,7 +693,7 @@ static void test_hashmap_get2(void) {
 }
 
 static unsigned long crippled_hashmap_func(const void *p, const uint8_t hash_key[HASH_KEY_SIZE]) {
-        return trivial_hash_func(p, hash_key) & 0xff;
+        return trivial_hash_func(INT_TO_PTR(PTR_TO_INT(p) & 0xff), hash_key);
 }
 
 static const struct hash_ops crippled_hashmap_ops = {