[yaml2obj][test] - Merge strtab-implicit-sections-*.yaml into...

[yaml2obj][test] - Merge strtab-implicit-sections-*.yaml into strtab-implicit-sections.yaml and improve testing of .shstrtab

This creates `strtab-implicit-sections.yaml` and merges 2 `strtab-implicit-sections*` tests into it.
I've also added a few tests for `.shstrtab` section related to section flags.

With that we have a single place where we can test implicit string table sections and
the `.shstrtab` section in particular.

Differential revision: https://reviews.llvm.org/D90372