[yaml2obj][obj2yaml][test] - Add base tests for relocation addends.

We had no test for `Addend` field of a relocation. Though the
current behavior is not ideal and might need to be fixed.

This patch adds 2 test cases to document the current
behavior and add a few FIXMEs. These FIXME are fixed in the
follow-up: https://reviews.llvm.org/D75527

Differential revision: https://reviews.llvm.org/D75528