DAOS-19340 test: set FI log level to warn for rebuild tests

[kccain]

For the daos_test rebuild tests, set engine environment variables
needed to capture warnings in logfiles for connection disruptions
(e.g., when engines stopped, restarted, and reintegrated). The goal is
especially to log when there are unusual connectivity disruptions
(e.g., after an engine restart, specific engine endpoint pairs unable
to communicate, often involving a common engine's single CaRT context).

Test-tag: daos_core_test_rebuild
Skip-unit-tests: true
Skip-fault-injection-test: true
Skip-test-rpms: true

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 'daos_test/rebuild.py:DaosCoreTestRebuild.test_rebuild_36 - timeout waiting for rebuild'
Status is 'Reopened'
Labels: '2.8.0rc2,ci_2.8_weekly,daily_test,verbs_provider'
https://daosio.atlassian.net/browse/DAOS-19340