lib: refactor lib/internal/fs/streams.js

Summary

Refactored lib/internal/fs/streams.js

• fixed if/else logic in ReadStream.prototype._read
• fixed argument variable name "callback" to "cb".
  In this file, other methods use "cb" for it.
• refactored some test files related to fs-stream.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines