elf: Add ld.so test with non-existing program name

None of the existing tests seem to cover the case where
_dl_signal_error is called without an active error handler.
The new elf/tst-rtld-does-not-exist test triggers such a
_dl_signal_error call from _dl_map_object.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>