scripted-diff: test: Use CTxMemPool in TestingSetup

After this commit, there should be no explicit instantiation of
CTxMemPool in src/test other than those in fuzz/ and setup_common

-BEGIN VERIFY SCRIPT-
find_regex="CTxMemPool\s+([^;({]+)(|\(\)|\{\});" \
    && git grep -l -E "$find_regex" -- src/test \
        | grep -v -e "^src/test/util/setup_common.cpp$" \
                  -e "^src/test/fuzz/" \
        | xargs sed -i -E "s@$find_regex@CTxMemPool\& \1 = *Assert(m_node.mempool);@g"
-END VERIFY SCRIPT-

src/test/validation_flush_tests.cpp

@@ -20,7 +20,7 @@ BOOST_FIXTURE_TEST_SUITE(validation_flush_tests, ChainTestingSetup)
 //!
 BOOST_AUTO_TEST_CASE(getcoinscachesizestate)
 {
-    CTxMemPool mempool;
+    CTxMemPool& mempool = *Assert(m_node.mempool);
     BlockManager blockman{};
     CChainState chainstate{&mempool, blockman, *Assert(m_node.chainman)};
     chainstate.InitCoinsDB(/*cache_size_bytes=*/1 << 10, /*in_memory=*/true, /*should_wipe=*/false);