tests,misc: Updated TestLib and boot-tests for gzipped imgs

tests,misc: Updated TestLib and boot-tests for gzipped imgs

In efforts to reduce storage costs and download times, the images hosted
by us have been gzipped. The TestLib framework has therefore been
extended to decompress gzipped files after download.

The x86-boot-tests are, at present, the only tests which use the gem5
images. These tests have been updated to download the gzipped image.

Change-Id: I6b2dbe9472a604148834820db8ea70e91e94376f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35257
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>