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-06-idna/run.sh

@@ -22,11 +22,11 @@ minidns_bg --addr=":9053" -zones=zones >> .minidns.log 2>&1
 
 CONFDIR=A generate_certs_for srv-ñ
 CONFDIR=A add_user ñangapirí@srv-ñ antaño
-CONFDIR=A add_user nadaA@nadaA nadaA
+CONFDIR=A add_user nadaa@nadaA nadaA
 
 CONFDIR=B generate_certs_for srv-ü
 CONFDIR=B add_user pingüino@srv-ü velóz
-CONFDIR=B add_user nadaB@nadaB nadaB
+CONFDIR=B add_user nadab@nadaB nadaB
 
 mkdir -p .logs-A .logs-B