test: allow slightly more diff in memory leak test

Previous limit was too low for debug builds.

Author: targos

test/parallel/test-crypto-dh-leak.js

@@ -26,4 +26,4 @@ const after = process.memoryUsage.rss();
 
 // RSS should stay the same, ceteris paribus, but allow for
 // some slop because V8 mallocs memory during execution.
-assert(after - before < 10 << 20, `before=${before} after=${after}`);
+assert(after - before < 10 << 21, `before=${before} after=${after}`);