In the context of some ol-gnus bug I created a minimal Gnus setup.  Ihor
wanted to have a look at it, so here it is.

To use:

- create an empty directory /foo/bar/baz and say

     unzip -qd /foo/bar/baz gnus-minimal-test.zip

- if you want to use the nnselect backend based on notmuch (haven't
   tested other search engines), ensure you have notmuch installed and
   generate the search index with

     HOME=/foo/bar/baz notmuch new

- HOME=/foo/bar/baz make vanilla, HOME=/foo/bar/baz emacs -Q, whatever.
   Only ensure to set the home directory to that newly created directory
   when starting Emacs.

- M-x load-file ~/.gnus RET

- M-x gnus RET

You should see a rather bland *Group* buffer along the following lines:

        1: nndraft:drafts
      182: nntp+news.gmane.io:gmane.emacs.announce
    156101: nntp+news.gmane.io:gmane.emacs.orgmode
        0: nnml+archive:test01
        0: nnml+archive:test02
        0: nnvirtual:test03
        0: nnselect:test04

(Please don't judge Gnus by this boring look - after all, this is a
minimal test!)

 From here you can select groups and articles with RET:

   nndrafts+ - where Gnus saves mail drafts
   nntp+...  - obvious
   nnml+...  - Gnus' own article archive backend
   nnvitual+ - "virtual" group on archives test01 & test02
   nnselect+ - search group searching for "emacs" in
               archives test01 & test02

and exit from these (and Gnus itself) with `q'.

More notes:

- Not sure as to how that could be used with ERT.  Gnus itself has only
   very basic tests in the Emacs distribution, AFAICT.  Using Gnus is
   something rather interactive, after all, and I have little ERT
   experience and none with interactive tests.

- Gnus expects all these files relative to the home directory.  In
   particular, there are some reference to that in the configuration files.
   Here it would be helpful if ERT could be tricked into changing the home
   directory to some temporary directory, as done in the instructions given
   above.  IIRC the home directory must be set before Gnus starts.

- If some of the groups shown above, like nntp (requiring internet
   connectivity) or nnselect (requiring notmuch), are not suitable for
   ERT, these could be removed from the setup.

- Gnus should be rather stable.  Anyhow, if you decide to test
   Org-Gnus-interaction, you most likely will be testing 10% Org
   functionality and 90% Gnus functionality.  Which comes at a certain
   risk, of course.

Have fun and feel free to ask or comment ...