testing: Make timing for TKM rekey scenarios a bit more stable

In particular for the first one randomization could trigger an additional
rekeying, which let the "Adding ESA ..." check fail.  But even without
randomization (could be seen in the second scenario that already uses
`rand_time=0`) 4 seconds can apparently be too low some time.

testing/tests/tkm/xfrmproxy-expire/pretest.dat

@@ -2,6 +2,8 @@ sun::systemctl start strongswan
 moon::rm /etc/swanctl/rsa/*
 moon::tkm_cfgtool -c /etc/tkm/tkm.conf -i /etc/swanctl/swanctl.conf -t /etc/tkm/tkm.bin -s /usr/local/share/tkm/tkmconfig.xsd
 moon::cat /etc/swanctl/swanctl.conf
+# disable randomization to avoid issues with our low rekey time
+moon::sed -i '/rekey_time=./arand_time=0' /etc/swanctl/swanctl.conf
 moon::tkm_keymanager -c /etc/tkm/tkm.bin -k /etc/tkm/moonKey.der -r /etc/tkm/strongswanCert.der:1 >/tmp/tkm.log 2>&1 &
 moon::expect-file /tmp/tkm.rpc.ike
 moon::service charon-tkm start