test: wait for the monitoring service to become active

test: wait for the monitoring service to become active

Otherwise we might start querying resolved too early, causing the
monitoring service to miss stuff:

```
[ 1103.149474] testsuite-75.sh[35]: + systemd-run -u resmontest.service -p Type=notify resolvectl monitor
[ 1103.353803] testsuite-75.sh[423]: Running as unit: resmontest.service
[ 1103.353989] testsuite-75.sh[35]: + knotc zone-begin test.
[ 1103.354160] testsuite-75.sh[425]: OK
...
[ 1103.355298] testsuite-75.sh[35]: + knotc reload
[ 1103.355363] testsuite-75.sh[438]: Reloaded
[ 1103.355536] testsuite-75.sh[35]: + : '--- nss-resolve/nss-myhostname tests'
[ 1103.355536] testsuite-75.sh[35]: + run getent -s resolve hosts ns1.unsigned.test
[ 1103.356127] testsuite-75.sh[443]: + getent -s resolve hosts ns1.unsigned.test
[ 1103.356505] testsuite-75.sh[444]: + tee /tmp/tmp.bXg5Uj5Jkk
[ 1103.359591] resolvectl[424]: → Q: ns1.unsigned.test IN AAAA
[ 1103.359591] resolvectl[424]: ← S: success
[ 1103.359850] testsuite-75.sh[444]: 10.0.0.1        ns1.unsigned.test
[ 1103.359939] resolvectl[424]: → Q: ns1.unsigned.test IN A
[ 1103.359939] resolvectl[424]: ← S: success
[ 1103.359939] resolvectl[424]: ← A: ns1.unsigned.test IN A 10.0.0.1
[ 1103.360149] testsuite-75.sh[35]: + grep -qE '^10\.0\.0\.1\s+ns1\.unsigned\.test' /tmp/tmp.bXg5Uj5Jkk
[ 1103.362119] systemd[1]: Starting resmontest.service...
[ 1103.362633] systemd[1]: Started resmontest.service.
[ 1103.363263] testsuite-75.sh[35]: + monitor_check_rr 'ns1.unsigned.test IN A 10.0.0.1'
[ 1103.363263] testsuite-75.sh[35]: + local 'match=ns1.unsigned.test IN A 10.0.0.1'
[ 1103.363377] testsuite-75.sh[35]: + set +o pipefail
[ 1103.363836] testsuite-75.sh[458]: + journalctl -u resmontest.service -f --full
[ 1103.364042] testsuite-75.sh[459]: + grep -m1 'ns1.unsigned.test IN A 10.0.0.1'
...
Trying to halt container. Send SIGTERM again to trigger immediate termination.
Container TEST-75 terminated by signal KILL.
```