plugin/cache: key cache on Checking Disabled (CD) bit (#6354)

* plugin/cache: key cache on Checking Disabled (CD) bit

Key the cache on CD bit, which effectively separates the entries for
queries with CD disabled or enabled.

Signed-off-by: Grant Spence <gspence@redhat.com>

plugin/test/helpers.go

@@ -29,15 +29,19 @@ func (p RRSet) Less(i, j int) bool { return p[i].String() < p[j].String() }
 // Case represents a test case that encapsulates various data from a query and response.
 // Note that is the TTL of a record is 303 we don't compare it with the TTL.
 type Case struct {
-	Qname             string
-	Qtype             uint16
-	Rcode             int
-	Do                bool
-	AuthenticatedData bool
-	Answer            []dns.RR
-	Ns                []dns.RR
-	Extra             []dns.RR
-	Error             error
+	Qname              string
+	Qtype              uint16
+	Rcode              int
+	Do                 bool
+	CheckingDisabled   bool
+	RecursionAvailable bool
+	AuthenticatedData  bool
+	Authoritative      bool
+	Truncated          bool
+	Answer             []dns.RR
+	Ns                 []dns.RR
+	Extra              []dns.RR
+	Error              error
 }
 
 // Msg returns a *dns.Msg embedded in c.