benchmark: enable assert benchmark with short len

`deepequal-typedarrays.js` throws if `len` is set to 100 or less due to
a hardcoded index. Calculate the index based on `len` so benchmark can
be run with small `len` values if desired.

PR-URL: #15174
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: Trott

benchmark/assert/deepequal-typedarrays.js

@@ -33,7 +33,8 @@ function main(conf) {
   const actual = new clazz(len);
   const expected = new clazz(len);
   const expectedWrong = Buffer.alloc(len);
-  expectedWrong[100] = 123;
+  const wrongIndex = Math.floor(len / 2);
+  expectedWrong[wrongIndex] = 123;
   var i;
 
   switch (conf.method) {