test: refactor test-preload

* assert.equal() -> assert.strictEqual()
* replace template string with a string; no variable substitution or
  concatenation or anything like that

PR-URL: https://github.com/nodejs/node/pull/9803


Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>