test: replace indexOf with includes

Replaced `indexOf` with `includes` in test-http-client-agent,
test-http-client-default-headers-exist

Refs: #12586

Author: thelostone-mc

test/parallel/test-http-client-agent.js

@@ -55,7 +55,8 @@ function request(i) {
     socket.on('close', function() {
       ++count;
       if (count < max) {
-        assert.strictEqual(http.globalAgent.sockets[name].indexOf(socket), -1);
+        assert.strictEqual(http.globalAgent.sockets[name].includes(socket),
+                           false);
       } else {
         assert(!http.globalAgent.sockets.hasOwnProperty(name));
         assert(!http.globalAgent.requests.hasOwnProperty(name));

test/parallel/test-http-client-default-headers-exist.js

@@ -46,9 +46,9 @@ const server = http.createServer(function(req, res) {
 
   const requestHeaders = Object.keys(req.headers);
   requestHeaders.forEach(function(header) {
-    assert.notStrictEqual(
-      expectedHeaders[req.method].indexOf(header.toLowerCase()),
-      -1,
+    assert.strictEqual(
+      expectedHeaders[req.method].includes(header.toLowerCase()),
+      true,
       `${header} should not exist for method ${req.method}`
     );
   });