test: support odd value for kStringMaxLength

V8 6.2 will support a larger maximum string length on 64-bit platforms.
Update the test to take into account its odd value ((1 << 30) - 1 - 24).

Refs: v8/v8@e8c9649
PR-URL: #14476
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>

test/addons/stringbytes-external-exceed-max/test-stringbytes-external-exceed-max-by-2.js

@@ -26,4 +26,4 @@ if (!binding.ensureAllocation(2 * kStringMaxLength))
   common.skip(skipMessage);
 
 const maxString = buf.toString('utf16le');
-assert.strictEqual(maxString.length, (kStringMaxLength + 2) / 2);
+assert.strictEqual(maxString.length, Math.floor((kStringMaxLength + 2) / 2));