test: fix flaky test-benchmark-fs

test-benchmark-fs uses common.tmpDir without first insuring it exists by
calling common.refreshTmpDir(). Add that function call.

PR-URL: https://github.com/nodejs/node/pull/17853


Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>