[Quarantine] test_clear_multiple_external_task_marker timing out

[potiuk]

It would be great to fix it before 2.0

Example failure here: https://github.com/apache/airflow/runs/1239113557?check_suite_focus=true#step:9:742

Apache Airflow version:

Kubernetes version (if you are using kubernetes) (use kubectl version):

Environment:

• Cloud provider or hardware configuration:
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:

What happened:

What you expected to happen:

How to reproduce it:

Anything else we need to know: