Replace std::experimental::filesystem with std::filesystem

Above c++17, std::filesystem is already supported, so replace
std::experimental::filesystem with std::filesystem.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I864bd80905e9b61428d17b20a91b3482198d20bc
diff --git a/test/physical.cpp b/test/physical.cpp
index c91fb94..ae831a3 100644
--- a/test/physical.cpp
+++ b/test/physical.cpp
@@ -10,7 +10,7 @@
 constexpr auto LED_OBJ = "/foo/bar/led";
 
 using Action = sdbusplus::xyz::openbmc_project::Led::server::Physical::Action;
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 fs::path create_sandbox()
 {