REGTESTS: add a test on "del backend"

Add a reg-tests to test "del backend" CLI command. First, checks are
performed to ensure a backend cannot be deleted if not in the expected
state.

Then, a "del backend" success is tested. Stats are dumped to ensure the
backend instance is indeed removed.

reg-tests/proxy/cli_del_backend.vtc

@@ -0,0 +1,58 @@
+varnishtest "Delete backend via cli"
+
+feature ignore_unknown_macro
+
+haproxy h1 -conf {
+	global
+	.if feature(THREAD)
+		thread-groups 1
+	.endif
+
+	defaults
+		mode http
+		timeout connect "${HAPROXY_TEST_TIMEOUT-5s}"
+		timeout client  "${HAPROXY_TEST_TIMEOUT-5s}"
+		timeout server  "${HAPROXY_TEST_TIMEOUT-5s}"
+
+	frontend fe
+		bind "fd@${feS}"
+		use_backend be_ref
+
+	listen li
+		bind "fd@${feli}"
+
+	backend be_ref
+
+	backend be
+		server s1 ${s1_addr}:${s1_port} disabled
+} -start
+
+haproxy h1 -cli {
+	send "experimental-mode on; del backend other"
+	expect ~ "No such backend."
+
+	send "experimental-mode on; del backend li"
+	expect ~ "Cannot delete a listen section."
+
+	send "experimental-mode on; del backend be_ref"
+	expect ~ "This proxy cannot be removed at runtime due to other configuration elements pointing to it."
+
+	send "show stat be 2 -1"
+	expect ~ "be,BACKEND,"
+
+	send "experimental-mode on; del backend be"
+	expect ~ "Backend must be unpublished prior to its deletion."
+
+	send "unpublish backend be;"
+	expect ~ ".*"
+	send "experimental-mode on; del backend be"
+	expect ~ "Only a backend without server can be deleted."
+
+	send "del server be/s1"
+	expect ~ ".*"
+	send "experimental-mode on; del backend be"
+	expect ~ "Backend deleted."
+
+	send "show stat be 2 -1"
+	expect !~ "be,BACKEND,"
+}