rtld/tests: Avoid function name conflict with libc opendir()

[arichardson]

Summary

This prevents these tests from being compiled with ASAN since the asan
interceptors also define opendir() but matching the libc function.

[oshogbo]

We have fdopendir in libc I wonder if this won't be confusing. Maybe we should rename this to something like rtld_opendir.
But whatever name your choose this LGTM.