slapos-sr-testing: Use libnetworkcache during test

Usually the slapos.core on nodes is installed with slapos.libnetworkcache,
so do the same for testing environment, so shacache can be used during
installation.

software/slapos-sr-testing/software.cfg

@@ -89,6 +89,7 @@ eggs =
   ${python-pynacl:egg}
   ${python-cryptography:egg}
   ${bcrypt:egg}
+  slapos.libnetworkcache
   slapos.core
   ${slapos.test.caddy-frontend-setup:egg}
   ${slapos.test.erp5-setup:egg}