test,base: Added GTest for base/loader/image_file_data.cc

test,base: Added GTest for base/loader/image_file_data.cc

image_file_data.cc reads an image file, or an image file compressed with
gzip. Mock image file data, and that data in a gzipped state, has been
included in base/loader/small_image_file.test.hh to aid testing.

Change-Id: I69691b93ca03c34d6bd736cbc5c6503115bd7b3f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22743
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>