The parallel behat runner script does not correctly handle the 'extraopts' passing; extra options / tags will not get a space between the additional CLI parameters, resulting in incorrect commands.

Example:
php admin/tool/behat/cli/run.php -v --tags '~@javascript' -f junit --out ../behat_results.xml -f progress --out php://stdout
Results in:
vendor/bin/behat -config /dev/shm/moodledata/behat1/behat/behat.yml -v '_{@javascript' -f 'junit' --out '../behat_results.xml' -f 'progress' --out 'php://stdout'}-tags="'@javascript'"

I will provide a small patch for this.