test: refactoring test-pipe-head

- Updated assert.equal to assert.strictEqual
- Updated 'var' to 'const'
- Using template literals

PR-URL: https://github.com/nodejs/node/pull/10036


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>