No core is produced when this failure occurs, either with debug or regular build.

I was able to reproduce this on x86 Bionic and Xenial VMs and on Cavium ThunderX:
==============================================================================
IPv4 Reassembly
==============================================================================
random order reassembly ERROR [ temp dir used by test case: /tmp/vpp-unittest-TestIPv4Reassembly-bQAsVB ]

==============================================================================
ERROR: random order reassembly
------------------------------------------------------------------------------
Traceback (most recent call last):
File "/home/jlinkes/vpp/test/test_reassembly.py", line 294, in test_random
packets = self.dst_if.get_capture(len(self.packet_infos))
File "/home/jlinkes/vpp/test/vpp_pg_interface.py", line 240, in get_capture
(len(capture.res), expected_count, name))
Exception: Captured packets mismatch, captured 31 packets, expected 257 packets on pg1

02:51:00,899 Symlink to failed testcase directory: /tmp/vpp-failed-unittests/vpp-unittest-TestIPv4Reassembly-bQAsVB-FAILED -> vpp-unittest-TestIPv4Reassembly-bQAsVB

==============================================================================
TEST RESULTS:
Scheduled tests: 1
Executed tests: 1
Passed tests: 0
Errors: 1
FAILURES AND ERRORS IN TESTS:
Testcase name: IPv4 Reassembly
ERROR: random order reassembly [test_reassembly.TestIPv4Reassembly.test_random]
==============================================================================

Some extra debugging with the attached test_reassembly.diff resulted in this:
==============================================================================
IPv4 Reassembly
==============================================================================
clear error

clear interfaces

error happened
show error
Count Node Reason

show interface
Name Idx State MTU (L3/IP4/IP6/MPLS) Counter Count
local0 0 down 0/0/0/0
pg0 1 up 9000/0/0/0 rx packets 3089
rx bytes 475910
ip4 3089
pg1 2 up 9000/0/0/0 tx packets 39
tx bytes 7268

clear error

clear interfaces

error happened
show error
Count Node Reason

show interface
Name Idx State MTU (L3/IP4/IP6/MPLS) Counter Count
local0 0 down 0/0/0/0
pg0 1 up 9000/0/0/0
pg1 2 up 9000/0/0/0

random order reassembly OK

==============================================================================

This corresponds with the attached log.txt and shows that only 3089 fragments out of 5953 are sent and only 39 packets are seemingly successfully reassembled. The attached TestIPv4Reassembly3.tgz is from this run.