test: Load genesis block to allow flush

This is needed to turn globals into member variables. Otherwise, this
will lead to issues:

runtime error: reference binding to null pointer of type 'CBlockFileInfo'
    #0 in std::vector<CBlockFileInfo, std::allocator<CBlockFileInfo> >::operator[](unsigned long) /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/bits/stl_vector.h:1046:2
    ElementsProject#1 in BlockManager::FlushBlockFile(bool, bool) src/node/blockstorage.cpp:540:47
    ElementsProject#2 in CChainState::FlushStateToDisk(BlockValidationState&, FlushStateMode, int) src/validation.cpp:2262:28
    ElementsProject#3 in CChainState::ResizeCoinsCaches(unsigned long, unsigned long) src/validation.cpp:4414:15
    ElementsProject#4 in validation_chainstate_tests::validation_chainstate_resize_caches::test_method() src/test/validation_chainstate_tests.cpp:66:12

src/test/validation_chainstate_tests.cpp

@@ -43,6 +43,7 @@ BOOST_AUTO_TEST_CASE(validation_chainstate_resize_caches)
     c1.InitCoinsDB(
         /*cache_size_bytes=*/1 << 23, /*in_memory=*/true, /*should_wipe=*/false);
     WITH_LOCK(::cs_main, c1.InitCoinsCache(1 << 23));
+    BOOST_REQUIRE(c1.LoadGenesisBlock()); // Need at least one block loaded to be able to flush caches
 
     // Add a coin to the in-memory cache, upsize once, then downsize.
     {