test_standalone: use buildout:newest=false in test profiles

Otherwise buildout will download latest version of eggs, which in the
case of setuptools now pulls version 45 which is no longer compatible
with python 2 and causes failures.
4 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed SlapOS.Eggs.UnitTest-Master.Python2

00:13:43

passed SlapOS.Eggs.UnitTest-Master.Python3

00:22:37

failed SlapOS.Eggs.UnitTest-Master.Python2

00:19:43

passed SlapOS.Eggs.UnitTest-Master.Python3

00:23:25