test: additional refactoring/cleanup of buffer 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 (buffer)

Description of change

Another round of cleanups for Buffer tests

• Favor use of strictEqual where possible
• Use const as appropriate
• Other miscellaneous cleanups