middleware/file: fix delegations (#376)

Fix the delegation handling in the *file* and *dnssec* middleware.
Refactor tests a bit and show that they are failling.

Add a Tree printer, cleanups and tests.
Fix wildcard test - should get no answer from empty-non-terminal

middleware/file/wildcard.go

@@ -0,0 +1,13 @@
+package file
+
+import "github.com/miekg/dns"
+
+// replaceWithWildcard replaces the left most label with '*'.
+func replaceWithAsteriskLabel(qname string) (wildcard string) {
+	i, shot := dns.NextLabel(qname, 0)
+	if shot {
+		return ""
+	}
+
+	return "*." + qname[i:]
+}