test: remove err timer from test-http-set-timeout

Removed the errorTimer from test-http-set-timeout.js, as this timer is
not necessary to test the setTimeout functionality.

Also edited the console.log message on line 8 to log the correct
timeout duration. Changed var to const, and added common.mustCall() to
on timeout and on error callbacks.

Fixes: https://github.com/nodejs/node/issues/9256
PR-URL: https://github.com/nodejs/node/pull/9264


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>