util: Store debug log file path in BCLog::Logger member.

This breaks the cyclic between logging and util.

src/test/test_bitcoin.cpp

@@ -47,7 +47,6 @@ BasicTestingSetup::BasicTestingSetup(const std::string& chainName)
         SetupNetworking();
         InitSignatureCache();
         InitScriptExecutionCache();
-        g_logger->m_print_to_file = false; // don't want to write to debug.log file
         fCheckBlockIndex = true;
         SelectParams(chainName);
         noui_connect();