Add missing link to library stdc++fs for test binaries

tests/CMakeLists.txt

@@ -103,6 +103,8 @@ target_link_libraries (unit_tests
   ${MPI_CXX_LINK_FLAGS} ${MPI_CXX_LIBRARIES}
   ${PETSC_LIBRARIES}
   Catch2
+  ${PUGS_STD_LINK_FLAGS}
+  stdc++fs
   )
 
 target_link_libraries (mpi_unit_tests
@@ -114,6 +116,8 @@ target_link_libraries (mpi_unit_tests
   ${MPI_CXX_LINK_FLAGS} ${MPI_CXX_LIBRARIES}
   ${PETSC_LIBRARIES}
   Catch2
+  ${PUGS_STD_LINK_FLAGS}
+  stdc++fs
   )
 
 enable_testing()