net: ibmveth: added KUnit tests for some buffer pool functions

Added KUnit tests for ibmveth_remove_buffer_from_pool and
ibmveth_rxq_get_buffer under new IBMVETH_KUNIT_TEST config option.

Signed-off-by: Dave Marquardt <davemarq@linux.ibm.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250501194944.283729-4-davemarq@linux.ibm.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>