test: fix another flaky stringbytes test

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
PR-URL: https://github.com/nodejs/node/pull/6073


Reviewed-By: Trott - Rich Trott <rtrott@gmail.com>