tests/net/socket/udp: Increase kernel heap size

On userspace platforms, this test needs a little bit of kernel heap.
The old mem_pool number was specified without metadata overhead
(i.e. it reflected 128 bytes of actual data available and the metadata
was stored silently somewhere else), where the new heap specifies the
size of the contiguous buffer in memory that stores both data and
chunk headers, etc...

Increase to 256 bytes.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>