Adapt tests to new rebootstrap method

Previously, the tests for reboostrapping with a version of Python
different than the original one had an expected failure to obtain
a distribution for setuptools, because the test environment blocks
access to PyPI etc.

However the new rebootstrap method uses the existing installation
to call buildout bootstrap with the new Python. This would usually
trigger fetching zc.buildout and dependencies online because the
already installed versions are tagged for the original Python, but
in the test the Python versions are actually the same and we merely
tricked rebootstrap into temporarily believing they are different
in order to trigger the desired code path, so the already installed
eggs are just directly reused.

slapos/rebootstrap/different_version.txt

@@ -75,7 +75,9 @@ is available, and buildout is using another python:
 Buildout will be restarted automatically to have this change applied.
 ************ REBOOTSTRAP: IMPORTANT NOTICE ************
 <BLANKLINE>
-While:
-  Installing.
-Error: Couldn't find a distribution for 'setuptools==...'
+Generated script '/sample-buildout/bin/buildout'.
+Develop: '/sample-buildout/recipes'
+Updating installpython.
+Installing realrun.
+Running with: /sample_buildout/parts/installpython/bin/python
 <BLANKLINE>