DAOS-18860 control: calc engine mem_size only on tgt count (#18407)

[tanabarr]

Pass memory size calculation to engine based on a 1gib/tgt quota
despite control-plane hugepage allocations taking MD-on-SSD
System-XStream into account when calculating.

Reorganize test cases in server_utils_test.go by splitting error
scenarios into a dedicated test function.

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 'rebuild/no_cap.py:RbldNoCapacity.test_rebuild_no_capacity - Failed to start servers after format'
Status is 'Awaiting backport'
Labels: 'ci_2.8_daily,daily_test'
Job should run at elevated priority (1)
https://daosio.atlassian.net/browse/DAOS-18860

[daosbuild3]

Test stage Unit Test completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18443/1/testReport/

[daosbuild3]

Test stage Functional Hardware Medium MD on SSD completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18443/1/testReport/

[daosbuild3]

Test stage NLT completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18443/3/testReport/

[daosbuild3]

Test stage Functional Hardware Medium MD on SSD completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-18443/3/testReport/

[daosbuild3]

Test stage Functional Hardware Medium Verbs Provider MD on SSD completed with status FAILURE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net//job/daos-stack/job/daos/view/change-requests/job/PR-18443/3/execution/node/1225/log

[tanabarr]

CI known failures:

• dfuse/daos_build_wb │ DAOS-18977
• test_daos_rebuild_ec DAOS-1311 build: Install dep in Dockerfiles #24 │ DAOS-18569
• REBUILD47/8 │ DAOS-17657