test: replace localhost with os.hostname in fs-readfilesync

Fixes: https://github.com/nodejs/node/issues/21501

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes (waiting for CI)
• tests and/or benchmarks are included
• commit message follows commit guidelines

@targos could you please try this on the setup mentioned in the issue if possible?