[VPP-1497] Parallel test execution produces failures on ARM in master

[vvalderrv]

HOW TO REPRODUCE: run make test-debug TEST_JOBS=4

The regular (non-parallel) run results in few reproducible failures. Parallel test run produces a much higher number of failures.

Most failures result in a core dump. The two attached console logs show that there is not much variation between the failures.

The core dumps are mostly from ACL or Classifier testcases.

Almost all cores are produced because of the same assert:
#10 0x0000ffff85401568 in vlib_buffer_advance (b=0xfffb4431f600, l=14) at /home/jlinkes/vpp/src/vlib/buffer.h:238
238 ASSERT ((b->flags & VLIB_BUFFER_NEXT_PRESENT) == 0 ||
(gdb) p b->flags
$1 = 262158

The flag is not always the same. In TestFIFReassembly, TestIPv4Reassembly, TestMACIP_IP4, TestMACIP_IP6 and TestMACIP it is 393222, otherwise it's 262158.

The two tests that don't fail because of this assert are TestL2fib and TestL2xc. This failure is documented in https://jira.fd.io/browse/VPP-1476