Reenable clear on TaskInstanceModelView for role User

The action was disable in apache#20659
which resolved apache#20655. The issue
only mentions that the edit is broken and should be disabled. So it seem
like the disabling of the clear action was unintentional.

Also based on the discussion in the PR
apache#20655 further reinforces this.
That the author believed it still worked could be explain by that using
a user with role `Admin` the action was still available and therefore
one could easily make a mistake believing it still worked as expected.

This PR reenables it action and modifies and existing test case to also
verify that clearing is possible using a user with the role `User`.

airflow/www/views.py

@@ -5378,6 +5378,7 @@ class TaskInstanceModelView(AirflowPrivilegeVerifierModelView):
     class_permission_name = permissions.RESOURCE_TASK_INSTANCE
     method_permission_name = {
         "list": "read",
+        "action_clear": "edit",
         "action_muldelete": "delete",
     }
     base_permissions = [

tests/www/views/test_views_tasks.py

@@ -706,26 +706,37 @@ def test_task_instance_delete_permission_denied(session, client_ti_without_dag_e
     assert session.query(TaskInstance).filter(TaskInstance.task_id == task_id).count() == 1
 
 
-def test_task_instance_clear(session, admin_client):
+@pytest.mark.parametrize(
+    "client_fixture, should_succeed",
+    [
+        ("admin_client", True),
+        ("user_client", True),
+        ("viewer_client", False),
+        ("anonymous_client", False),
+    ],
+)
+def test_task_instance_clear(session, request, client_fixture, should_succeed):
+    client = request.getfixturevalue(client_fixture)
     task_id = "runme_0"
+    initial_state = State.SUCCESS
 
     # Set the state to success for clearing.
     ti_q = session.query(TaskInstance).filter(TaskInstance.task_id == task_id)
-    ti_q.update({"state": State.SUCCESS})
+    ti_q.update({"state": initial_state})
     session.commit()
 
     # Send a request to clear.
     rowid = _get_appbuilder_pk_string(TaskInstanceModelView, ti_q.one())
-    resp = admin_client.post(
+    resp = client.post(
         "/taskinstance/action_post",
         data={"action": "clear", "rowid": rowid},
         follow_redirects=True,
     )
-    assert resp.status_code == 200
+    assert resp.status_code == (200 if should_succeed else 404)
 
     # Now the state should be None.
     state = session.query(TaskInstance.state).filter(TaskInstance.task_id == task_id).scalar()
-    assert state == State.NONE
+    assert state == (State.NONE if should_succeed else initial_state)
 
 
 def test_task_instance_clear_failure(admin_client):