test: fix another flaky stringbytes test
Checklist
-
tests and code linting passes -
a test and/or benchmark is included -
the commit message follows commit guidelines
Affected core subsystem(s)
test
Description of change
Avoid depending on precise timing of when an object will be collected by GC. This test was missed by #6039 as it happened to be in a different directory than the rest.
Ref: https://github.com/nodejs/node/pull/6039 R=@trott CI: https://ci.nodejs.org/job/node-test-pull-request/2172/