Stabilize flaky test r1813

Because it uses hard-coded ports, it often appears as failing in vtest
so now it should succeed if the address wasn't available.

bin/varnishtest/tests/r01813.vtc

 varnishtest "Duplicate -a arguments"
 
-shell -err -expect {have same address} "varnishd -d -a 127.0.0.1:38484 -a 127.0.0.1:38484 -b localhost:80 2>&1"
+shell -err -match "(have same address)|(already in use)" {
+	varnishd -d -a 127.0.0.1:38484 -a 127.0.0.1:38484 -b localhost:80
+}