test: fix process.title expectation

[thefourtheye]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

process.title would work properly only in FreeBSD, OSX, and Linux as
per test/parallel/test-setproctitle.js.

This patch makes sure that the test expects an empty string in other
platforms.

This patch helps fix the SmartOS failures in
https://ci.nodejs.org/job/node-test-commit/6962/ for
#10456

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cc @Trott