Document prerequisites on Testing

[a4lg]

pyelftools is required for a helper script.

This commit adds basic installation guide to install pyelftools and removes "automatic installation" of pyelftools from Makefile.

This is based on the counter proposal to my #1374+#1376 by @cmuellner (#1374 (comment)).

[a4lg]

Note that I didn't touch .github directory because setup-apt.sh already contains installation of python3-pyelftools.

[pz9115]

I remember that if we want to run dejagnu, we also need tcl and expect, but they are not in the prerequisite, can you also add them here, thanks.

see: https://www.gnu.org/software/dejagnu/

[a4lg]

@pz9115 I see. I think what we really need is expect alone (tcl or its libraries should be a dependency of expect) and check whether this is correct.

[cmuellner]

In general, this PR looks good.

Two comments:

• RedHat/Fedora switched from yum to dnf long ago.
• I would have put that text into the prerequisites section on top. I consider testing a fundamental feature of this repo, so all prerequisites for testing are prerequisites for this repo.

[kito-cheng]

Thanks!