test: refactor test-http-after-connect.js

- Replace assert.equal() to assert.strictEqual()
- Replace var with const where applicable
- Removed firstBodyChunk which is never used
- Remove the process.on('exit', ...) and replace its functionality by
- Using common.mustCall() where applicable

PR-URL: https://github.com/nodejs/node/pull/10229


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>