Unit test doublespends in new blocks

As suggested by Greg Maxwell-- unit test to make sure a block
with a double-spend in it doesn't pass validation if half of
the double-spend is already in the memory pool (so full-blown
transaction validation is skipped) when the block is received.

src/Makefile.test.include

@@ -73,6 +73,7 @@ BITCOIN_TESTS =\
   test/test_bitcoin.h \
   test/timedata_tests.cpp \
   test/transaction_tests.cpp \
+  test/txvalidationcache_tests.cpp \
   test/uint256_tests.cpp \
   test/univalue_tests.cpp \
   test/util_tests.cpp