Update PythonTest.h to support building using GCC v7

This commit fixes the error reported below by copying and adapting
compatibility logic already used in source file like PluginManager.cpp

Error:

[ 78%] Built target Sofa.Component.LinearSystem_test
In file included from /path/to/Slicer-SOFA-build/SofaPython3/Testing/src/SofaPython3Testing/PythonTestExtractor.h:22:0,
                 from /path/to/Slicer-SOFA-build/SofaPython3/Testing/src/SofaPython3Testing/PythonTestExtractor.cpp:21:
/path/to/Slicer-SOFA-build/SofaPython3/Testing/src/SofaPython3Testing/PythonTest.h:28:12: error: ‘std::filesystem’ has not been declared
 using std::filesystem::path;
            ^~~~~~~~~~

Author: jcfr

Testing/src/SofaPython3Testing/PythonTest.h

@@ -23,6 +23,16 @@
 #include <string>
 #include <SofaPython3Testing/config.h>
 #include <sofa/testing/BaseTest.h>
+#if __has_include(<filesystem>)
+  #include <filesystem>
+#elif __has_include(<experimental/filesystem>)
+  #include <experimental/filesystem>
+  namespace std {
+      namespace filesystem = experimental::filesystem;
+  }
+#else
+  error "Missing the <filesystem> header."
+#endif
 
 #if __has_include(<filesystem>)
   #include <filesystem>