[llvm-readobj][test] - Test "Format" values.

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