Replace includes of boost/filesystem.h with fs.h

This is step one in abstracting the use of boost::filesystem.

src/test/test_bitcoin.cpp

@@ -7,6 +7,7 @@
 #include "chainparams.h"
 #include "consensus/consensus.h"
 #include "consensus/validation.h"
+#include "fs.h"
 #include "key.h"
 #include "validation.h"
 #include "miner.h"
@@ -24,7 +25,6 @@
 
 #include <memory>
 
-#include <boost/filesystem.hpp>
 #include <boost/thread.hpp>
 
 FastRandomContext insecure_rand_ctx(true);