app/test: fix missing NULL pointer checks

app/test: fix missing NULL pointer checks

In test_sched, we are missing NULL pointer checks after create_mempool()
and rte_pktmbuf_alloc(). Add in these checks using TEST_ASSERT_NOT_NULL macros.

VERIFY macro was removed and replaced by standard test ASSERTS from "test.h" header.
This provides additional information to track when the failure occurred.

Signed-off-by: Daniel Mrzyglod <danielx.t.mrzyglod@intel.com>