[llvm-readelf/obj] - Refine the error message about the broken string table.

This:
1) Changes `reportWarning` to `reportUniqueWarning` (no-op here).
2) Adds more context to the message.
3) Merges `broken-dynsym-link.test` into `dyn-symbols.test`, adds more testing.

Differential revision: https://reviews.llvm.org/D92380