alternator test: drastically reduce time to boot Scylla
alternator test: drastically reduce time to boot Scylla

The alternator test, test/alternator/run, runs Scylla and runs the
various tests against it. Before this patch, just booting Scylla took
about 26 seconds (for a dev build, on my laptop). This patch reduces
this delay to less than one second!

It turns out that almost the entire delay was artificial, two periods
of 12 seconds "waiting for the gossip to settle", which are completely
unnecessary in the one-node cluster used in the Alternator test.
So a simple "--skip-wait-for-gossip-to-settle 0" parameter eliminates
these long delays completely.

Amusingly, the Scylla boot is now so fast, that I had to change a "sleep 2"
in the test script to "sleep 1", because 2 seconds is now much more than
it takes to boot Scylla :-)

Fixes #6310.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20200428145035.22894-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla