unit_tests: fix minor blockchain_db regression

Due to the change in ordering for adding block and tx data
to the database in f2986ccfc1,
adding a block twice now throws TX_EXISTS, not BLOCK_EXISTS.

tests/unit_tests/blockchain_db.cpp

@@ -288,7 +288,7 @@ TYPED_TEST(BlockchainDBTest, AddBlock)
   ASSERT_TRUE(compare_blocks(this->m_blocks[0], b));
 
   // assert that we can't add the same block twice
-  ASSERT_THROW(this->m_db->add_block(this->m_blocks[0], t_sizes[0], t_diffs[0], t_coins[0], this->m_txs[0]), BLOCK_EXISTS);
+  ASSERT_THROW(this->m_db->add_block(this->m_blocks[0], t_sizes[0], t_diffs[0], t_coins[0], this->m_txs[0]), TX_EXISTS);
 
   for (auto& h : this->m_blocks[0].tx_hashes)
   {