It also implicitly tested the xlog parallel write slots thing, as that is common code to all recovery.
During development, I hit a lot of bugs in that patch by setting wal_buffers to 32kb (the minimum). Causes more backends to wait for each other, exposing deadlocks.
I've run the foreign key version with 32kb for a while and nothing turned up. I should probably run the gist or gin versions, as they should put more stress on the volume of WAL generated.