acceptance: parser_functions/puppet_lookup_cmd.rb test fails when multiple agents in beaker config

The parser_functions/puppet_lookup_cmd.rb test fails when run in the Puppet Server CI pipeline:

https://jenkins.puppetlabs.com/view/puppet-server/view/all/job/platform_puppet-server_integration-system_no-conditional_full-master/139/LAYOUT=redhat7-64ma-windows2008r2-64a,LDAP_TYPE=default,PLATFORM=default,label=beaker/testReport/junit/%28root%29/ruby_puppet_acceptance_tests_parser_functions/puppet_lookup_cmd_rb/

The root cause of the failure appears to be that the test was written to only work when a single `agent` is in the CI/beaker configuration:

This line...

https://github.com/puppetlabs/puppet/blob/92fa18efefa40fa8ef0b6a44d6d900f9e522c45a/acceptance/tests/parser_functions/puppet_lookup_cmd.rb#L14

...has:

@agentname = on(agent, facter('fqdn')).stdout.chomp

I think the test needs to be reworked to iterate through each agent the configuration with an agents.each and do the appropriate steps.