test: refactor test-fs-readfile-unlink

• Use tmp directory instead of mutating the fixtures directory.
• Add comment explaining that the unlinkSync() at the end of the test is part of the test. Otherwise it may be tempting to remove it as unnecessary tmp directory cleanup.

Checklist

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

Affected core subsystem(s)

test fs