fixup! Make software/seleniumserver a minimal software again

testnodes also needs Xvfb, which is provided by xserver

software/seleniumrunner/software.cfg

 # Selenium runner, helper software for erp5testnode.
 #
-# Seleniumrunner responsability is to install a default firefox as ${buildout:bin-directory}/firefox and
-# a geckodriver as ${buildout:bin-directory}/geckodriver for erp5testnode.
+# Seleniumrunner responsability is to install Xvfb as
+# ${buildout:parts-directory}/xserver/bin/Xvfb, a default firefox as
+# ${buildout:bin-directory}/firefox and a geckodriver as
+# ${buildout:bin-directory}/geckodriver for erp5testnode.
 
 [buildout]
 extends =
@@ -14,6 +16,7 @@ extends =
 parts =
    slapos-cookbook
    template
+   xserver
    firefox-wrapper
    geckodriver