Stop DynaLoader.t from unload File::Glob

Stop DynaLoader.t from unload File::Glob

File::Glob now sets PL_opfreehook.  That means that if we unload it we
will leave PL_opfreehook pointing to an invalid address, resulting in
a crash when ops are freed.

I didn’t catch this because the tests are skipped on darwin.  They
don’t actually need to be, at least under recent versions, so I modi-
fied the skip code.