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/Makefile.am.include b/test/Makefile.am.include
index 98cc6fb..74da526 100644
--- a/test/Makefile.am.include
+++ b/test/Makefile.am.include
@@ -14,7 +14,6 @@
 	$(PHOSPHOR_DBUS_INTERFACES_CXXFLAGS) \
 	$(CODE_COVERAGE_CXXFLAGS)
 AM_LDFLAGS = \
-	-lstdc++fs \
 	$(SDBUSPLUS_LIBS) \
 	$(PHOSPHOR_DBUS_INTERFACES_LIBS) \
 	$(CODE_COVERAGE_LIBS) \
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()
 {
diff --git a/test/sysfs.cpp b/test/sysfs.cpp
index e8339e7..7a65b94 100644
--- a/test/sysfs.cpp
+++ b/test/sysfs.cpp
@@ -24,7 +24,7 @@
 
 #include <gtest/gtest.h>
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 constexpr unsigned long MAX_BRIGHTNESS_VAL = 128;