encoding: make Block convertible to span

Change-Id: Idb501467d1c56b95e7fdb76acda63b919fe98f85
diff --git a/tests/unit/util/io.t.cpp b/tests/unit/util/io.t.cpp
index ebe5799..270270c 100644
--- a/tests/unit/util/io.t.cpp
+++ b/tests/unit/util/io.t.cpp
@@ -187,10 +187,7 @@
   void
   wireDecode(const Block& block)
   {
-    // block must be 0xBB, 0x01, 0xEE
-    BOOST_CHECK_EQUAL(block.type(), 0xBB);
-    BOOST_REQUIRE_EQUAL(block.value_size(), 1);
-    BOOST_CHECK_EQUAL(block.value()[0], 0xEE);
+    BOOST_TEST(block == "BB01EE"_block);
   }
 };