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: nodejs#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>

test/parallel/test-http-status-reason-invalid-chars.js

@@ -7,7 +7,7 @@ const net = require('net');
 
 function explicit(req, res) {
   assert.throws(() => {
-    res.writeHead(200, `OK\r\nContent-Type: text/html\r\n`);
+    res.writeHead(200, 'OK\r\nContent-Type: text/html\r\n');
   }, /Invalid character in statusMessage/);
 
   assert.throws(() => {
@@ -20,7 +20,7 @@ function explicit(req, res) {
 
 function implicit(req, res) {
   assert.throws(() => {
-    res.statusMessage = `OK\r\nContent-Type: text/html\r\n`;
+    res.statusMessage = 'OK\r\nContent-Type: text/html\r\n';
     res.writeHead(200);
   }, /Invalid character in statusMessage/);
   res.statusMessage = 'OK';
@@ -40,8 +40,8 @@ const server = http.createServer((req, res) => {
   let left = 2;
   const check = common.mustCall((res) => {
     left--;
-    assert.notEqual(res.headers['content-type'], 'text/html');
-    assert.notEqual(res.headers['content-type'], 'gotcha');
+    assert.notStrictEqual(res.headers['content-type'], 'text/html');
+    assert.notStrictEqual(res.headers['content-type'], 'gotcha');
     if (left === 0) server.close();
   }, 2);
   http.get(`${url}/explicit`, check).end();