[lld][ELF][test] Expand testing of symbols in mergeable sections

Whilst reviewing some internal testing, I noticed a couple of holes in
coverage of mergeable sections containing symbols. This patch addresses
these holes:
1) Show that mid-piece symbols have their values updated properly when
   pieces are merged.
2) Show the behaviour of symbols in mergeable pieces when --gc-sections
   is enabled.

Reviewed by: grimar, MaskRay

Differential Revision: https://reviews.llvm.org/D86543