Skip tests for dl_unload_file() when built with -DPERL_GLOBAL_STRUCT_PRIVATE

Skip tests for dl_unload_file() when built with -DPERL_GLOBAL_STRUCT_PRIVATE

Building with -DPERL_GLOBAL_STRUCT_PRIVATE uses alternative implementations
of Perl_my_cxt_init() and Perl_my_cxt_index() which store the pointer to the
extension name. For a dynamically loaded shared object this is a pointer to
memory which will be released if that shared object is unloaded, meaning that
any *other* module which calls Perl_my_cxt_index() will likely attempt a read
of unmapped memory.