test: update test-buffer-alloc to use tpl literals

Updated a single usage of string concatenation to use template literals.
There are other usages of string concatenation in this file, but they
are used to make test code more readable.

test/parallel/test-buffer-alloc.js

@@ -310,12 +310,12 @@ assert.strictEqual('TWFu', (Buffer.from('Man')).toString('base64'));
   assert.strictEqual(quote, b.toString('ascii', 0, quote.length));
 
   // check that the base64 decoder ignores whitespace
-  const expectedWhite = expected.slice(0, 60) + ' \n' +
-                        expected.slice(60, 120) + ' \n' +
-                        expected.slice(120, 180) + ' \n' +
-                        expected.slice(180, 240) + ' \n' +
-                        expected.slice(240, 300) + '\n' +
-                        expected.slice(300, 360) + '\n';
+  const expectedWhite = `${expected.slice(0, 60)}
+                         ${expected.slice(60, 120)}
+                         ${expected.slice(120, 180)}
+                         ${expected.slice(180, 240)}
+                         ${expected.slice(240, 300)}
+                         ${expected.slice(300, 360)}`;
   b = Buffer.allocUnsafe(1024);
   bytesWritten = b.write(expectedWhite, 0, 'base64');
   assert.strictEqual(quote.length, bytesWritten);