test: validate errors in test-buffer-indexof

* validate errors in assert.throws
* use arrow functions

PR-URL: nodejs#10752
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-buffer-indexof.js

@@ -337,15 +337,20 @@ assert.strictEqual(Buffer.from('aaaaa').indexOf('b', 'ucs2'), -1);
   }
 }
 
-assert.throws(function() {
-  b.indexOf(function() { });
-});
-assert.throws(function() {
+const argumentExpected =
+    /^TypeError: "val" argument must be string, number or Buffer$/;
+
+assert.throws(() => {
+  b.indexOf(() => { });
+}, argumentExpected);
+
+assert.throws(() => {
   b.indexOf({});
-});
-assert.throws(function() {
+}, argumentExpected);
+
+assert.throws(() => {
   b.indexOf([]);
-});
+}, argumentExpected);
 
 // All code for handling encodings is shared between Buffer.indexOf and
 // Buffer.lastIndexOf, so only testing the separate lastIndexOf semantics.