Describe the bug
#793 (557f08c) introduced a non-acceptable regression: Unit tests can no longer be run in parallel.

We have a script that runs unit tests independently as separate processes and in parallel in contrib/devtools/run-unit-tests.sh. I've binary searched through the commit history and built until I arrived at 557f08c which apparently breaks this.

I've verified:

74d598a – okay
a8a002f – okay
ba3ebd4 – broken
557f08c – broken
cc3bb51 (parent of 557f08c) – okay

To Reproduce
Run contrib/devtools/run-unit-tests.sh.

Expected behavior
All tests should pass.

Environment
Ubuntu 18.04 using gcc, macOS using clang

Additional context
none.