add more file based tests

[peshay]

• assert_file_owner to test if a user is owner of a file
• assert_file_permission to test permissions set on a file
• assert_not_zero if a file is greater 0 byte
• assert_file_set_group_id if set-group-ID is set
• assert_file_set_user_id if set-user-ID is set
• assert_sticky_bit if sticky bit is set on file

[peshay]

#16