TESTS: checks: add a simple test config for external checks

ext-check.cfg tests both for success and failure in two different backends.

tests/ext-check.cfg

@@ -0,0 +1,26 @@
+global
+	maxconn 500
+	external-check
+	stats socket /tmp/sock1 mode 666 level admin
+
+defaults
+	timeout client 5s
+	timeout server 5s
+	timeout connect 5s
+	mode http
+
+listen stats
+	bind :8888
+	stats uri /
+
+listen up
+	bind :8001
+	option external-check
+	external-check command /bin/true
+	server srv 127.0.0.1:8000 check inter 1000
+
+listen down
+	bind :8002
+	option external-check
+	external-check command /bin/false
+	server srv 127.0.0.1:8000 check inter 1000