use calculated size for overflow tests

This greatly reduces how much these tests depend on hard-wired knowledge
about the size classes.

test/simple-memory-corruption/eight_byte_overflow_large.c

@@ -7,7 +7,8 @@ OPTNONE int main(void) {
     if (!p) {
         return 1;
     }
-    *(p + 256 * 1024 + 7) = 0;
+    size_t size = malloc_usable_size(p);
+    *(p + size + 7) = 0;
     free(p);
     return 0;
 }

test/simple-memory-corruption/eight_byte_overflow_small.c

@@ -7,7 +7,8 @@ OPTNONE int main(void) {
     if (!p) {
         return 1;
     }
-    *(p + 8 + 7) ^= 1;
+    size_t size = malloc_usable_size(p);
+    *(p + size + 7) ^= 1;
     free(p);
     return 0;
 }