Increase limit for network space overhead test

This test imposes a limit on the average bytes of space per chunk for
network traffic. However this number depends on VM implementation
details, and upcoming changes to V8's array buffer management require a
small bump to this limit in this test.

test/sequential/test-net-bytes-per-incoming-chunk-overhead.js

diff --git a/test/sequential/test-net-bytes-per-incoming-chunk-overhead.js b/test/sequential/test-net-bytes-per-incoming-chunk-overhead.js
index 8f766e8..c2275b5 100644
--- a/test/sequential/test-net-bytes-per-incoming-chunk-overhead.js
+++ b/test/sequential/test-net-bytes-per-incoming-chunk-overhead.js
@@ -37,5 +37,5 @@
   const bytesPerChunk =
     (process.memoryUsage().rss - baseRSS) / receivedChunks.length;
   // We should always have less than one page (usually ~ 4 kB) per chunk.
-  assert(bytesPerChunk < 512, `measured ${bytesPerChunk} bytes per chunk`);
+  assert(bytesPerChunk < 600, `measured ${bytesPerChunk} bytes per chunk`);
 });