Using Ansible variables in testinfra

Using TestInfra with Ansible backend for testing purposes. Everything goes fine except using Ansible itself while running tests

test.py

import pytest
def test_zabbix_agent_package(host):
    package = host.package("zabbix-agent")
    assert package.is_installed
    package_version = host.ansible("debug", "msg=")["msg"]
    (...)

where zabbix_agent_version is an Ansible variable from group_vars. It can be obtained by running this playbook

- hosts: all
  become: true
  tasks:
  - name: debug
    debug: msg=

command executing tests

pytest --connection=ansible --ansible-inventory=inventory  --hosts=$hosts -v test.py

ansible.cfg

[defaults]
timeout = 10
host_key_checking = False
library=library/
retry_files_enabled = False
roles_path=roles/
pipelining=true
ConnectTimeout=60
remote_user=deploy
private_key_file=/opt/jenkins/.ssh/deploy

the output I get is

self = <ansible>, module_name = 'debug', module_args = 'msg=', check = True, kwargs = {}
result = {'failed': True, 'msg': "the field 'args' has an invalid value, which appears to include a variable that is undefined. The error was: 'zabbix_agent_version' is undefined"}

    def __call__(self, module_name, module_args=None, check=True, **kwargs):
        if not self._host.backend.HAS_RUN_ANSIBLE:
            raise RuntimeError((
                "Ansible module is only available with ansible "
                "connection backend"))
        result = self._host.backend.run_ansible(
            module_name, module_args, check=check, **kwargs)
        if result.get("failed", False) is True:
>           raise AnsibleException(result)
E           AnsibleException: Unexpected error: {'failed': True,
E            'msg': u"the field 'args' has an invalid value, which appears to include a variable that is undefined. The error was: 'zabbix_agent_version' is undefined"}

/usr/lib/python2.7/site-packages/testinfra/modules/ansible.py:70: AnsibleException

Any idea why Ansible can't see this variable when running testinfra's Ansible module while it can see it while running Ansible alone?