test: Reduce main sources of overhead in integration tests

The integration tests spend a lot of time on some ancilliary actions,
which slows them down: generating certificates, adding users, and
waiting for things to happen.

To improve the performance of those actions, this patch:

- Makes (most) tests use plain passwords (-20%)
- Adds a certificate cache to reuse certs (-10%)
- Tightens the sleep loops (-5%)

In aggregate, this patch results in a speedup of the integration tests
of ~30-40%.

Note that some of the tests required adjusting the username, because
`chasquid-util user-add` would convert them to lowercase as per PRECIS
mapping.

test/t-13-reload/run.sh

@@ -13,7 +13,7 @@ generate_certs_for testserver
 #
 
 # Start with the user with the wrong password, and no aliases.
-add_user someone@testserver password111
+chasquid-util-user-add someone@testserver password111
 rm -f config/domains/testserver/aliases
 
 mkdir -p .logs
@@ -28,7 +28,7 @@ fi
 
 # Change password, add an alias; then wait a bit more than the reload period
 # and try again.
-add_user someone@testserver password222
+chasquid-util-user-add someone@testserver password222
 echo "analias: someone" > config/domains/testserver/aliases
 sleep 0.2