Test suite: fix running with XDG_SESSION_TYPE unset (refs: #17596)

I think this check makes things worse that if were not there,
for two reasons.

1. Most users of our test suite run it with sudo, which filters out
   XDG_SESSION_TYPE from the environment by default, so regardless of whether
   they're using a Wayland desktop, if they try using --view, a developer using
   sudo will see this error:

     ./run_test_suite: line 273: XDG_SESSION_TYPE: unbound variable

   … which:

    - is less informative for Wayland users than the error this check was meant
      to replace;

    - breaks stuff for folks who don't use Wayland.

2. Even under GNOME + Wayland, when running the test suite under sudo,
   it turns out that without this check, --view works fine:
   xtigervncviewer does not error out merely because it detects Wayland,
   and AFAICT it does its job :)