[llvm-readobj][llvm-readelf][test] - Add a test to check how we dump relocation addends.

Seems we do not test how we print relocation addends well.
And the behavior of dumpers does not seem to be ideal here
(and llvm-readelf does not match GNU as the test case shows).

This patch adds a test case to document the current behavior.

Differential revision: https://reviews.llvm.org/D75671