[llvm-readobj] - Refine --needed-libs implementation and add a test.

We have no good test for --needed-libs option.
The one we have as a part of Object/readobj-shared-object.test
is not complete.

In this patch I've did a minor NFC changes to the implementation and
added a test. This allowed to remove this piece from
Object/readobj-shared-object.test

Differential revision: https://reviews.llvm.org/D73174