use std::filesystem in File tests

Summary: Use `std::filesystem` or `std::experimental::filesystem`, via `folly/portability/Filesystem.h`, in `folly/test/FileTest.cpp`. Replaces the use of `boost::filesystem`.

Reviewed By: Orvid

Differential Revision: D26572916

fbshipit-source-id: f769e00b2772f3bfaed899f5694d89db87089e35

folly/test/FileTest.cpp

@@ -19,18 +19,16 @@
 #include <fstream>
 #include <random>
 
-#include <boost/filesystem.hpp>
 #include <fmt/core.h>
 #include <glog/logging.h>
 
 #include <folly/String.h>
 #include <folly/portability/Fcntl.h>
+#include <folly/portability/Filesystem.h>
 #include <folly/portability/GTest.h>
 
 using namespace folly;
 
-namespace fs = boost::filesystem;
-
 namespace {
 
 void expectWouldBlock(ssize_t r) {