test: add tests for clearBuffer state machine

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

stream, test

Description of change

This checks to see that clearBuffer appropriately decrements the correct values in _writableState when clearBuffer is invoked in end.

Closes #8687 (closed).