**Is your feature request related to a problem? Please describe.**

Warning on building using g++ 8.3.

```
 1 In file included from ./txdb.h:11,
 2                  from ./validation.h:26,
 3                  from ./test/util/chainstate.h:13,
 4                  from test/validation_chainstate_tests.cpp:11:
 5 ./chain.h: In member function ‘void validation_chainstate_tests::chainstate_update_tip::test_method()’:
 6 ./chain.h:422:27: warning: ‘background_cs’ may be used uninitialized in this function [-Wmaybe-uninitialized]
 7          return vChain.size() > 0 ? vChain[vChain.size() - 1] : nullptr;
 8                 ~~~~~~~~~~~^~
 9 test/validation_chainstate_tests.cpp:109:18: note: ‘background_cs’ was declared here
10      CChainState* background_cs;
11                   ^~~~~~~~~~~~~
12
```

**Describe the solution you'd like**

Able to build without warnings.

**Describe alternatives you've considered**

N/A.

**Additional context**

`cd src/ && make test/test_bitcoin-validation_chainstate_tests.o`

Bisecting this warning, I ended up in revision 673a5bd3377929a0
by @jamesob.

Thank you very much and please excuse the noise.

#include <chainparams.h>
#include <random.h>
#include <uint256.h>
#include <consensus/validation.h>
#include <sync.h>
#include <rpc/blockchain.h>
#include <test/util/chainstate.h>
#include <test/util/setup_common.h>
#include <validation.h>
#include <vector>
#include <boost/test/unit_test.hpp>
BOOST_FIXTURE_TEST_SUITE(validation_chainstate_tests, TestingSetup)
BOOST_AUTO_TEST_CASE(validation_chainstate_resize_caches)
{
    ChainstateManager manager;
    WITH_LOCK(::cs_main, manager.m_blockman.m_block_tree_db = std::make_unique<CBlockTreeDB>(1 << 20, true));
    CTxMemPool mempool;
    
    auto add_coin = [](CCoinsViewCache& coins_view) -> COutPoint {
        Coin newcoin;
        uint256 txid = InsecureRand256();
        COutPoint outp{txid, 0};
        newcoin.nHeight = 1;
        newcoin.out.nValue = InsecureRand32();
        newcoin.out.scriptPubKey.assign((uint32_t)56, 1);
        coins_view.AddCoin(outp, std::move(newcoin), false);
        return outp;