switch experimental::fs to std::fs

std::filesystem was added to C++17 and is well supported now.  Remove
the older std::experimental::filesystem usage.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I6d1ef0f3ac02954ddbffc332393fa11930dffad7
diff --git a/vpnor/test/tmpd.hpp b/vpnor/test/tmpd.hpp
index ae6da9e..38fb428 100644
--- a/vpnor/test/tmpd.hpp
+++ b/vpnor/test/tmpd.hpp
@@ -12,7 +12,7 @@
 
 #include <cassert>
 #include <cstring>
-#include <experimental/filesystem>
+#include <filesystem>
 #include <fstream>
 #include <vector>
 
@@ -23,7 +23,7 @@
 namespace test
 {
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 class VpnorRoot
 {