test(dnsserver): improve core/dnsserver test coverage (#7317)

Add comprehensive tests for multiple components including server blocks
inspection, configuration handling, DoH/DoQ writers, and server startup
functions. Increases overall test coverage from 27% to 38.4% with
particular focus on register.go, https.go, quic.go, and config.go.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

core/dnsserver/https_test.go

@@ -88,3 +88,72 @@ func TestDoHWriter_Request(t *testing.T) {
 		})
 	}
 }
+
+func TestDoHWriter_Write(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   []byte
+		wantErr bool
+	}{
+		{
+			name: "valid DNS message",
+			// A minimal valid DNS query message
+			input: []byte{
+				0x00, 0x01, /* ID */
+				0x01, 0x00, /* Flags: query, recursion desired */
+				0x00, 0x01, /* Questions: 1 */
+				0x00, 0x00, /* Answer RRs: 0 */
+				0x00, 0x00, /* Authority RRs: 0 */
+				0x00, 0x00, /* Additional RRs: 0 */
+				0x03, 'w', 'w', 'w',
+				0x07, 'e', 'x', 'a', 'm', 'p', 'l', 'e',
+				0x03, 'c', 'o', 'm',
+				0x00,       /* Null terminator for domain name */
+				0x00, 0x01, /* Type: A */
+				0x00, 0x01, /* Class: IN */
+			},
+			wantErr: false,
+		},
+		{
+			name:    "empty message",
+			input:   []byte{},
+			wantErr: true, // Expect an error because unpacking an empty message will fail
+		},
+		{
+			name:    "invalid DNS message",
+			input:   []byte{0x00, 0x01, 0x02}, // Truncated message
+			wantErr: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			d := &DoHWriter{}
+			n, err := d.Write(tt.input)
+
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Write() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if !tt.wantErr && n != len(tt.input) {
+				t.Errorf("Write() bytes written = %v, want %v", n, len(tt.input))
+			}
+			if !tt.wantErr && d.Msg == nil {
+				t.Errorf("Write() d.Msg is nil, expected a parsed message")
+			}
+		})
+	}
+}
+
+func TestDoHWriter_Close(t *testing.T) {
+	d := &DoHWriter{}
+	if err := d.Close(); err != nil {
+		t.Errorf("Close() error = %v, want nil", err)
+	}
+}
+
+func TestDoHWriter_TsigStatus(t *testing.T) {
+	d := &DoHWriter{}
+	if err := d.TsigStatus(); err != nil {
+		t.Errorf("TsigStatus() error = %v, want nil", err)
+	}
+}