tests: improve Block test suite

* Reorder test cases to follow function order in block.hpp.
* Move prependVarNumber and prependNonNegativeInteger tests
  to EncodingBuffer test suite.
* Add test for Block() default constructor.
* Add test for Block(uint32_t) constructor.
* Add test for Block::parse function.

refs #4171

Change-Id: Ia4eaf36f6ced9c2dabc7531ba541a5a2f92bae89
diff --git a/src/encoding/block.cpp b/src/encoding/block.cpp
index 82b1110..768b624 100644
--- a/src/encoding/block.cpp
+++ b/src/encoding/block.cpp
@@ -27,11 +27,11 @@
 #include "encoding-buffer.hpp"
 #include "tlv.hpp"
 
-#include <boost/lexical_cast.hpp>
 #include <boost/asio/buffer.hpp>
 
 namespace ndn {
 
+BOOST_CONCEPT_ASSERT((boost::EqualityComparable<Block>));
 #if NDN_CXX_HAVE_IS_NOTHROW_MOVE_CONSTRUCTIBLE
 static_assert(std::is_nothrow_move_constructible<Block>::value,
               "Block must be MoveConstructible with noexcept");