[Object/yaml2obj/obj2yaml][test] - Split, cleanup and move MIPS abi-flags.yaml test. NFCI.

We have the `Object/Mips/abi-flags.yaml` which tests how yaml2obj/obj2yaml
handle `SHT_MIPS_ABIFLAGS` sections.

This patch splits it into two tests: one for obj2yaml and one for yaml2obj
and moves the result to right places.

Differential revision: https://reviews.llvm.org/D88231