HDDS-10225. Speed up TestSCMHAManagerImpl

[adoroszlai]

What changes were proposed in this pull request?

TestSCMHAManagerImpl has three test cases, each starting new leader SCM from scratch:

• add SCM
• remove SCM
• invalid scenarios

[INFO] Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 47.041 s - in org.apache.hadoop.hdds.scm.ha.TestSCMHAManagerImpl

This change reduces test execution time by starting a single leader SCM.

1. Testing removal of peer requires adding it first, so testRemoveSCM already depends on "add SCM" functionality working correctly. The patch deletes "add SCM" setup from testRemoveSCM, now it relies on the state left by testAddSCM.
2. Testing invalid scenarios does not need a fresh SCM leader, requests being tested are invalid either way.

Also, some additional code cleanup, performed in separate commits for easier review.

https://issues.apache.org/jira/browse/HDDS-10225

How was this patch tested?

[INFO] Tests run: 3, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 18.489 s - in org.apache.hadoop.hdds.scm.ha.TestSCMHAManagerImpl

CI:
https://github.com/adoroszlai/ozone/actions/runs/7678209884

[nandakumar131]

Thanks @adoroszlai for the improvement!

[adoroszlai]

Thanks @nandakumar131 for reviewing and merging this.