[llvm-readobj]Add additional testing for various ELF features

This patch adds testing of areas of the code that are not fully tested,
in particular dynamic table printing, ELF type printing, handling of
edge cases where things are missing/empty (relocations/program header
tables/section header table), and the --string-dump switch.

Reviewed by: grimar, higuoxing, rupprecht

Differential Revision: https://reviews.llvm.org/D58677

llvm-svn: 355003