tests: dynamic_thread: fix test_thread_index_management

This test was working by accident onarm64 and riscv64. Those
architectures have large register files, even more so considering
their 64-bit nature.

This test works by calling k_object_alloc(K_OBJ_THREAD) until thread
index exhaustion. However here it exhausted heap memory before running
out of thread indexes. There was a test to make sure that wasn't the
case by attempting a k_malloc(256). But here that succeeded just
because 256 is far smaller than a struct k_thread on the above
architectures.

Fix this by:

- attempting an additional allocation with the actual object size
  instead of an arbitrary 256 bites
- increasing the heap size as 8192 was clearly insufficient for the
  above platforms.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>