Try to merge tail with next block when splitting

When splitting a memory block, check if the next block is free.
If it is, then just extend it upwards instead of creating a new block.
This fixes a bug where when shrinking existing allocations would result in irreversible free space fragmentation.

When testing on the host, test all the poisoning configurations.

components/heap/multi_heap_poisoning.c

@@ -244,6 +244,8 @@ void *multi_heap_realloc(multi_heap_handle_t heap, void *p, size_t size)
        place.)
 
        For now we just malloc a new buffer, copy, and free. :|
+
+       Note: If this ever changes, multi_heap defrag realloc test should be enabled.
     */
     size_t orig_alloc_size = head->alloc_size;