Re-enable FreeBSD workaround for mysterious test failures.

extras/build-tests/nix/run-tests

@@ -3,6 +3,14 @@
 # This script fires up a two-server IRC network and runs the test framework
 #
 
+if [[ "$OSTYPE" == "freebsd"* ]]; then
+	# Workaround for strange FBSD 12 issue:
+	# if using -P20 tests randomly fail with
+	# class variables missing. -P2 seems to
+	# work, but -P1 is the safest option:
+	export MAXPARALLEL=1
+fi
+
 # Exit on error:
 set -e