DAOS-18659 test: Adjusting pool scm size

[saurabhtandan]

The test fails intermittently as the available pool scm size in mdonssd mode is slightly less than the available ram size. Reducing the pool scm size.

Test-tag: test_snapshot_aggregation
Test-repeat: 5

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'container/snapshot_aggregation.py:SnapshotAggregation.test_snapshot_aggregation - ENOMEM error during pool creation failed with MD-on-SSD'
Status is 'In Progress'
Labels: 'md_on_ssd'
https://daosio.atlassian.net/browse/DAOS-18659