[llvm-readobj] - Merge `gnu-symbols.test` to `symbols.test` and cleanup.

This cleans up and merges `gnu-symbols.test` to `symbols.test`.
Initially `gnu-symbols.test` tested the following things:
1) How symbols are printed in GNU style.
   It does not make sense to have a separate file for such tests.
2) It tried to test proc-specific symbol indexes. The test was incomplete and
   also we already have `symbol-shndx.test` for that, so this part was removed.
3) It tested `--dyn-symbols` and `--symbols` correlation. All following
   cases were moved to `symbols.test`:
   a) That `--dyn-symbols` does not trigger showing regular symbols..
   b) That `--symbols` triggers `--dyn-symbols` implicitly.
   c) That `--dyn-symbols` and `--symbols` works fine together.

Differential revision: https://reviews.llvm.org/D71697