tests: make `cp` python >= 3.3 compatible

> Changed in version 3.3: Added follow_symlinks argument. Now returns path
> to the newly created file.

slapos/rebootstrap/tests.py

@@ -97,7 +97,9 @@ parts =
 def setUp(test):
   zc.buildout.testing.buildoutSetUp(test)
   zc.buildout.testing.install_develop('slapos.rebootstrap', test)
-  test.globs['cp'] = shutil.copy
+  def cp(src, dst):
+    shutil.copy(src, dst)
+  test.globs['cp'] = cp
   _setUp(**test.globs)
 
 def test_suite():