test: improve test-buffer-includes.js

* verify error message
* use arrow funcs

PR-URL: nodejs/node#11203
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-buffer-includes.js

@@ -273,15 +273,17 @@ for (let lengthIndex = 0; lengthIndex < lengths.length; lengthIndex++) {
   }
 }
 
-assert.throws(function() {
-  b.includes(function() { });
-});
-assert.throws(function() {
+const expectedError =
+  /^TypeError: "val" argument must be string, number or Buffer$/;
+assert.throws(() => {
+  b.includes(() => {});
+}, expectedError);
+assert.throws(() => {
   b.includes({});
-});
-assert.throws(function() {
+}, expectedError);
+assert.throws(() => {
   b.includes([]);
-});
+}, expectedError);
 
 // test truncation of Number arguments to uint8
 {