test: refactor/cleanup a number of cluster tests

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test, cluster

Description of change

Some general refactoring / improvements to various cluster related tests:

• Move shared code into common
• Favor use of strictEqual
• Add some missing common.mustCalls
• Other general cleanup