test: fix `fs.rmdir` deprecation warning

[erickzhao]

(node:23706) [DEP0147] DeprecationWarning: In future versions of Node.js, fs.rmdir(path, { recursive: true }) will be removed. Use fs.rm(path, { recursive: true }) instead
(Use `node --trace-deprecation ...` to show where the warning was created)

Node 16: https://nodejs.org/api/deprecations.html#DEP0147

Supersedes #133