test: refine test-http-status-reason-invalid-chars

* replace unneeded template strings with strings; there was no variable
  substitution or concatenation or anything like that

* assert.notEqual() -> assert.notStrictEqual()

PR-URL: https://github.com/nodejs/node/pull/9802


Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>