Finding the file and unittest that triggers an ICE during dub project build only when unittests are enabled

[Per Nordlöw]

When I build my project as

     dub run --compiler=dmd --build=unittest

it crashes as

Performing "unittest" build using dmd for x86_64.
phobos-next ~master: building configuration "library"...
Segmentation fault (core dumped)
dmd failed with exit code 139.

whereas

     dub run --compiler=dmd --build=debug

passes.

How do I most easily track down which unittest in which file that 
causes the crash?