test: add check for wrk to test-keep-alive

test/pummel/test-keep-alive.js requires `wrk` to be installed. Check if
it is, and skip the test if it isn't.

This is yet another step in preparation for running pummel tests in CI
daily.

PR-URL: https://github.com/nodejs/node/pull/25516


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>