[master] (re)enabled an MX test with fixing test data.

I'm not sure why this has been disabled until now, but it seems to me
just about test data preparation.  so I fixed it that way.
should be quite trivial, and test actually passed.  directly pushing,
skipping review.

src/lib/dns/tests/rdata_mx_unittest.cc

@@ -74,12 +74,9 @@ TEST_F(Rdata_MX_Test, toWireRenderer) {
 TEST_F(Rdata_MX_Test, toWireBuffer) {
     renderer.writeName(Name("example.com"));
     rdata_mx.toWire(obuffer);
-}
 
-TEST_F(Rdata_MX_Test, DISABLED_toWireBuffer) {
-// XXX: does not pass
     vector<unsigned char> data;
-    UnitTestUtil::readWireData("rdata_mx_toWire1", data);
+    UnitTestUtil::readWireData("rdata_mx_toWire2", data);
     EXPECT_PRED_FORMAT4(UnitTestUtil::matchWireData, obuffer.getData(),
                         obuffer.getLength(), &data[0], data.size());
 }

src/lib/dns/tests/testdata/rdata_mx_toWire2

@@ -0,0 +1,12 @@
+#
+# compressed MX RDATA stored in an output buffer
+#
+# sentinel name: example.com.
+# 0  1  2  3  4  5  6  7  8  9 10  1  2 (bytes)
+#(7) e  x  a  m  p  l  e (3) c  o  m  .
+ 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00
+# PREFERENCE: 10
+  00 0a
+# EXCHANGE: not compressed
+#(4) m  x ptr=0
+ 02 6d 78 07 65 78 61 6d 70 6c 65 03 63 6f 6d 00