ansible-test - ensure trailing separator is added for connection unit test target (#74176)

For connection units tests, if the `__init__.py` file is modified, the trailing separator was
not added, making the target invalid.

changelogs/fragments/ansible-test-connection-units-init.yml

@@ -0,0 +1,4 @@
+bugfixes:
+  - >-
+    ansible-test - ensure the correct unit test target is given when the
+    ``__init__.py`` file is modified inside the connection plugins directory

test/lib/ansible_test/_internal/classification.py

@@ -527,7 +527,7 @@ class PathMapper:
                     'integration': self.integration_all_target,
                     'windows-integration': self.integration_all_target,
                     'network-integration': self.integration_all_target,
-                    'units': units_dir,
+                    'units': os.path.join(units_dir, ''),
                 }
 
             units_path = os.path.join(units_dir, 'test_%s.py' % name)