[obj2yaml][test] - Improve and fix section-group.yaml test.

It has multiple issues fixed by this patch:
1) It shouldn't test how llvm-readelf/yaml2obj works.
2) It should use "-NEXT" prefix for check lines.
3) It can use YAML macros, that allows to use a single YAML.
4) It should probably test the case when a group member is a null section.

Differential revision: https://reviews.llvm.org/D93753