Unittests for uint256.h

Unit tests for uint256.h. The file uint160_tests.cpp is no longer
needed. The ad-hoc tests which were in uint256.h are also no longer
needed. The new tests achieve 100% coverage.

src/test/Makefile.am

@@ -34,7 +34,7 @@ test_bitcoin_SOURCES = alert_tests.cpp \
   key_tests.cpp miner_tests.cpp mruset_tests.cpp multisig_tests.cpp \
   netbase_tests.cpp pmt_tests.cpp rpc_tests.cpp script_P2SH_tests.cpp \
   script_tests.cpp serialize_tests.cpp sigopcount_tests.cpp test_bitcoin.cpp \
-  transaction_tests.cpp uint160_tests.cpp uint256_tests.cpp util_tests.cpp \
+  transaction_tests.cpp uint256_tests.cpp util_tests.cpp \
   sighash_tests.cpp $(JSON_TEST_FILES) $(RAW_TEST_FILES)
 
 if ENABLE_WALLET