CI: ansible_mitogen tests use deprecated yaml callback plugin

[moreati]

The yaml plugin is now deprecated, superceded by callback_result_format = yaml. However the exact output may have changed, and the Mitogen integration tests rely on the output text.

Suggest something like this, with test updates

--- a/tests/ansible/ansible.cfg
+++ b/tests/ansible/ansible.cfg
@@ -1,5 +1,7 @@
 [defaults]
 any_errors_fatal = true
+# Ansible >= 6 (ansible-core >= 2.13), replaces stdout_callback = yaml
+callback_result_format = yaml
 # callbacks_enabled was added in Ansible 4 (ansible-core 2.11).
 # profile_tasks: Displays timing for each task and summary table of top N tasks
 # timer: Displays "Playbook run took 0 days, 0 hours, ..."
@@ -17,7 +19,6 @@ strategy_plugins = ../../ansible_mitogen/plugins/strategy
 inventory_plugins = lib/inventory
 action_plugins = lib/action
 callback_plugins = lib/callback
-stdout_callback = yaml
 vars_plugins = lib/vars
 library = lib/modules
 filter_plugins = lib/filters
diff --git a/tox.ini b/tox.ini
index 9f3be10f..d8fe5434 100644
--- a/tox.ini
+++ b/tox.ini
@@ -114,6 +114,7 @@ setenv =
     PIP_CONSTRAINT={toxinidir}/tests/constraints.txt
     # Print warning on the first occurence at each module:linenno in Mitogen. Available Python 2.7, 3.2+.
     PYTHONWARNINGS=default:::ansible_mitogen,default:::mitogen
+    ansible{2.10,3,4,5}: ANSIBLE_STDOUT_CALLBACK=yaml
     # Ansible 6 - 8 (ansible-core 2.13 - 2.15) require Python 2.7 or >= 3.5 on targets
     ansible6: MITOGEN_TEST_DISTRO_SPECS=centos7 centos8 debian9 debian10 debian11 ubuntu1604 ubuntu1804 ubuntu2004
     ansible7: MITOGEN_TEST_DISTRO_SPECS=centos7 centos8 debian9 debian10 debian11 ubuntu1604 ubuntu1804 ubuntu2004

related to #1284