fix(modules/gitlab_runner): Fix exception in check mode on new runners (#10918)

* fix(modules/gitlab_runner): Fix exception in check mode on new runners

When a new runner is added in check mode, the role used to throw an
exception. Fix this by returning a valid runner object instead of a
boolean.

Fixes #8854

* docs: Add changelog fragment

changelogs/fragments/10918-gitlab-runner-fix-check-mode.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - gitlab_runner - fix exception in check mode when a new runner is created (https://github.com/ansible-collections/community.general/issues/8854).

plugins/modules/gitlab_runner.py

@@ -343,7 +343,10 @@ class GitLabRunner(object):
     '''
     def create_runner(self, arguments):
         if self._module.check_mode:
-            return True
+            class MockRunner:
+                def __init__(self):
+                    self._attrs = {}
+            return MockRunner()
 
         try:
             if arguments.get('token') is not None: