test: change tmp directories prefix

`.tmp` prefix allows easier exclusion

PR-URL: https://github.com/nodejs/node/pull/16372
Refs: https://github.com/nodejs/node/pull/16010
Refs: https://github.com/nodejs/node/issues/16278


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>