Update checkpointing test for async

[anfals]

Description

XLML checkpointing tests only test the synchronous checkpointing path; this updates the test to allow the option for async checkpointing, which is the path recommended for customers

Tests

This is a test update

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed.