test: scale keepalive timeouts for slow machines

The test was using fixed timeouts and that seems to be causing sporadic
test failures on pi1-docker host (which is a very slow machine.)

Fixes: https://github.com/nodejs/node/issues/30828

PR-URL: https://github.com/nodejs/node/pull/30834


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>