test: fix minor stream-writableState linter error

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test

Description of change

A minor linter error was introduced in a recent commit.

/Users/gib/wrk/com/DANGER/node/test/parallel/test-stream-writableState-ending.js
  3:7  error  'common' is assigned a value but never used  no-unused-vars

Refs: https://github.com/nodejs/node/pull/8707

cc/ @mcollina