net: tests: Unref net_buf using net_nbuf_unref

Using net_buf_unref() technically works but debugging the network buffer
allocations is more difficult if done like that.

Change-Id: Ieaa0978aa69e2f0baa924fe842ca1f116c348ef1
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>