system-test: increase timeout for keepalive tests from 1s to 2s

The keepalive tests were occasionally bumping up against
the 1s timeout we had set for curl, but a 1s 99.9% tail
latency is a little agressive.  Drop it to 2s to make the
tests less flaky.

What we really need is a proper load testing setup, so
the keepalive test isn't doing double-duty.