fix(dns): retry configurable resolvers

Treat resolver transport failures as inconclusive checks so they do not open outage alerts.

checks/cdns/config_test.go

@@ -0,0 +1,26 @@
+package cdns
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestResolversFromEnv(t *testing.T) {
+	t.Setenv("RSMON_DNS_RESOLVERS", " 188.93.17.19, 2001:db8::53, invalid ")
+
+	got := resolversFromEnv()
+	want := []string{"188.93.17.19", "2001:db8::53"}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("resolversFromEnv() = %v, want %v", got, want)
+	}
+}
+
+func TestResolversFromEnvFallsBackToDefaults(t *testing.T) {
+	t.Setenv("RSMON_DNS_RESOLVERS", "not-an-address")
+
+	got := resolversFromEnv()
+	want := []string{"188.93.17.19", "188.93.16.19", "82.117.245.122"}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("resolversFromEnv() = %v, want %v", got, want)
+	}
+}