test: increase slop limit in memory leak test

It transpires that the extra bookkeeping in debug builds sometimes makes
the increase in RSS go _just_ over the 5 MB limit, by fewer than 100 kB.
Double the limit so we hopefully don't run into it any time again soon.

The memory leak it tests for was one where RSS grew by hundreds of
megabytes over the lifetime of the test; 5 vs. 10 MB is insignificant.

Fixes: #21076

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

@@ -23,4 +23,4 @@ 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 < 5 << 20, `before=${before} after=${after}`);
+assert(after - before < 10 << 20, `before=${before} after=${after}`);