Use `rmm::percent_of_free_device_memory` in arena test

[wence-]

Description

Rather than hand-coding a fraction of the device memory use the utility routine.

• Closes #1674

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

[wence-]

/merge