This adds testing for the "Format" field printed with `--file-headers`.
llvm-readelf doesn't use them, so only llvm-readobj needs to be tested.
All possible values are defined and tested in `ELFObjectFile<ELFT>::getFileFormatName()`.
Here we test just a few arbitrary ones.
Differential revision: https://reviews.llvm.org/D86350
Georgii Rymar