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-14-tls_tracking/run.sh

@@ -20,10 +20,10 @@ minidns_bg --addr=":9053" -zones=zones >> .minidns.log 2>&1
 # B - listens on :2025, hosts srv-B
 
 CONFDIR=A generate_certs_for srv-A
-CONFDIR=A add_user userA@srv-A userA
+CONFDIR=A add_user usera@srv-A userA
 
 CONFDIR=B generate_certs_for srv-B
-CONFDIR=B add_user userB@srv-B userB
+CONFDIR=B add_user userb@srv-B userB
 
 rm -rf .data-A .data-B .mail .certs
 mkdir -p .logs-A .logs-B .mail .certs