Fix Node 16 deprecation

[niksy]

Node 16 shows following deprecation:

(node:81356) [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)

Since fs.rmSync was added in v14.14.0, and yeoman-test supports Node 12, calls to fs.rmSync are conditionally checked.