flaky test parallel/test-blob in debug mode

Example in CI: https://ci.nodejs.org/job/node-test-commit-linux-containered/24844/nodes=ubuntu1804_sharedlibs_debug_x64/testReport/

Fails ~40% of the runs on my machine:

```
Command: out/Debug/node /home/mzasso/git/nodejs/node/test/parallel/test-blob.js
--- CRASHED (Signal: 4) ---
[00:27|% 100|+  60|-  40]: Done
```

/cc @jasnell