upstream CI: run PR tests only for affected plugins

The current workflow for bug fixing or new enhancements in
ansible-freeipa includes running Ansible playbooks tests for all the
available plugins for every pull request, even for contained
modifications.

This patch creates a new workflow for pull requests where only the
affected plugins are tested in the PR. Changes that might affect other
parts of the code will trigger tests for the parts affected.

A utility script, utils/filter_tests, is used to set the variables
IPA_ENABLED_MODULES and IPA_ENABLED_TESTS before executing the tests,
effectively limiting which tests are executed. The script uses the
python plugin 'utils/filter_plugins.py' which lists all test modules
that should be executed for a list of modified source files.

Tests are selected for execution based on the plugin name. For example,
a change to 'plugins/modules/ipalocation.py' would trigger all playbook
tests under 'tests/location'. If a test playbook is modified, it is
scheduled to be executed. Changes to any file under
'plugins/module_utils' will force the execution of all tests, since any
module might be affected by that change.

The nature of the change is not evaluated, so a simple typo fix of a
comment in a file under 'plugins/module_utils' would still schedule all
test playbooks to be executed.

For roles, any file changed under the role directory would set the role
to be included in the tests. Playbook tests for roles must be created
under 'tests/<rolename>_role', where role name in the name of the role
without 'ipa', for example, the 'ipabackup' role test playbooks would
be stored under 'tests/backup_role'.

Since there is the possibility that the list of tests to be executed
might be less than the number of tests groups used (3), a new pytest
dependency was added, pytest-custom_exit_code, so that having no tests
to run isn't a test failure.

A new pipeline on Azure needs to be created to use the new test script.