test/cmake: set C++11 standard for tensor

unittest/CMakeLists.txt

@@ -31,6 +31,9 @@ endmacro(ADD_LIB_UNIT_TEST)
 add_lib_unit_test(matrix)
 if(EIGENPY_WITH_TENSOR_SUPPORT)
   add_lib_unit_test(tensor)
+  if(NOT DEFINED CMAKE_CXX_STANDARD)
+    set_target_properties(tensor PROPERTIES CXX_STANDARD 11)
+  endif()
 endif()
 add_lib_unit_test(geometry)
 add_lib_unit_test(complex)