DAOS-16983 test: reduce dfuse/caching_check threshold (#17486)

[daltonbohning]

Reduce expected threshold from 300% to 250%.
Discard performance of first read with caching enabled. Also misc test cleanup.

Test-tag: test_dfuse_caching_check
Test-repeat: 10
Skip-unit-tests: true
Skip-fault-injection-test: 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 'dfuse/caching_check.py:DfuseCachingCheck.test_dfuse_caching_check - Expected a speedup of 3 but got 2.8059'
Status is 'Awaiting backport'
Labels: 'ci_master_weekly,weekly_test'
Job should run at elevated priority (1)
https://daosio.atlassian.net/browse/DAOS-16983

[daosbuild3]

Test stage Functional Hardware Medium completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-17538/1/testReport/